Named after  Arthur Wieferich 

Publication year  1909 
Author of publication  Wieferich, A. 
No. of known terms  2 
Conjectured no. of terms  Infinite 
Subsequence of 

First terms  1093, 3511 
Largest known term  3511 
OEIS index  A001220 
In number theory, a Wieferich prime is a prime number p such that p^{2} divides 2^{p − 1} − 1,^{[4]} therefore connecting these primes with Fermat's little theorem, which states that every odd prime p divides 2^{p − 1} − 1. Wieferich primes were first described by Arthur Wieferich in 1909 in works pertaining to Fermat's Last Theorem, at which time both of Fermat's theorems were already well known to mathematicians.^{[5]}^{[6]}
Since then, connections between Wieferich primes and various other topics in mathematics have been discovered, including other types of numbers and primes, such as Mersenne and Fermat numbers, specific types of pseudoprimes and some types of numbers generalized from the original definition of a Wieferich prime. Over time, those connections discovered have extended to cover more properties of certain prime numbers as well as more general subjects such as number fields and the abc conjecture.
As of April 2022^{[update]}, the only known Wieferich primes are 1093 and 3511 (sequence A001220 in the OEIS).
The stronger version of Fermat's little theorem, which a Wieferich prime satisfies, is usually expressed as a congruence relation 2^{p 1} ≡ 1 (mod p^{2}). From the definition of the congruence relation on integers, it follows that this property is equivalent to the definition given at the beginning. Thus if a prime p satisfies this congruence, this prime divides the Fermat quotient . The following are two illustrative examples using the primes 11 and 1093:
Wieferich primes can be defined by other equivalent congruences. If p is a Wieferich prime, one can multiply both sides of the congruence 2^{p−1} ≡ 1 (mod p^{2}) by 2 to get 2^{p} ≡ 2 (mod p^{2}). Raising both sides of the congruence to the power p shows that a Wieferich prime also satisfies 2^{p2} ≡2^{p} ≡ 2 (mod p^{2}), and hence 2^{pk} ≡ 2 (mod p^{2}) for all k ≥ 1. The converse is also true: 2^{pk} ≡ 2 (mod p^{2}) for some k ≥ 1 implies that the multiplicative order of 2 modulo p^{2} divides gcd(p^{k} − 1, φ(p^{2})) = p − 1, that is, 2^{p−1} ≡ 1 (mod p^{2}) and thus p is a Wieferich prime. This also implies that Wieferich primes can be defined as primes p such that the multiplicative orders of 2 modulo p and modulo p^{2} coincide: ord_{p2} 2 = ord_{p} 2, (By the way, ord_{1093}2 = 364, and ord_{3511}2 = 1755).
H. S. Vandiver proved that 2^{p−1} ≡ 1 (mod p^{3}) if and only if .^{[7]}^{: 187 }
In 1902, Meyer proved a theorem about solutions of the congruence a^{p − 1} ≡ 1 (mod p^{r}).^{[8]}^{: 930 }^{[9]} Later in that decade Arthur Wieferich showed specifically that if the first case of Fermat's last theorem has solutions for an odd prime exponent, then that prime must satisfy that congruence for a = 2 and r = 2.^{[10]} In other words, if there exist solutions to x^{p} + y^{p} + z^{p} = 0 in integers x, y, z and p an odd prime with p ∤ xyz, then p satisfies 2^{p − 1} ≡ 1 (mod p^{2}). In 1913, Bachmann examined the residues of . He asked the question when this residue vanishes and tried to find expressions for answering this question.^{[11]}
The prime 1093 was found to be a Wieferich prime by W. Meissner for all primes p < 2000 and found this residue to be zero for t = 364 and p = 1093, thereby providing a counterexample to a conjecture by Grave about the impossibility of the Wieferich congruence.^{[12]} E. Haentzschel later ordered verification of the correctness of Meissner's congruence via only elementary calculations.^{[13]}^{: 664 } Inspired by an earlier work of Euler, he simplified Meissner's proof by showing that 1093^{2}  (2^{182} + 1) and remarked that (2^{182} + 1) is a factor of (2^{364} − 1).^{[14]} It was also shown that it is possible to prove that 1093 is a Wieferich prime without using complex numbers contrary to the method used by Meissner,^{[15]} although Meissner himself hinted at that he was aware of a proof without complex values.^{[12]}^{: 665 }
in 1913 and confirmed to be the only such prime below 2000. He calculated the smallest residue ofThe prime 3511 was first found to be a Wieferich prime by N. G. W. H. Beeger in 1922^{[16]} and another proof of it being a Wieferich prime was published in 1965 by Guy.^{[17]} In 1960, Kravitz^{[18]} doubled a previous record set by Fröberg ^{[19]} and in 1961 Riesel extended the search to 500000 with the aid of BESK.^{[20]} Around 1980, Lehmer was able to reach the search limit of 6×10^{9}.^{[21]} This limit was extended to over 2.5×10^{15} in 2006,^{[22]} finally reaching 3×10^{15}. It is now known that if any other Wieferich primes exist, they must be greater than 6.7×10^{15}.^{[23]}
In 2007–2016, a search for Wieferich primes was performed by the distributed computing project Wieferich@Home.^{[24]} In 2011–2017, another search was performed by the PrimeGrid project, although later the work done in this project was claimed wasted.^{[25]} While these projects reached search bounds above 1×10^{17}, neither of them reported any sustainable results.
In 2020, PrimeGrid started another project that searches for Wieferich and Wall–Sun–Sun primes simultaneously. The new project uses checksums to enable independent doublechecking of each subinterval, thus minimizing the risk of missing an instance because of faulty hardware.^{[26]} The project ended in December 2022, definitely proving that a third Wieferich prime must exceed 2^{64} (about 18×10^{18}).^{[27]}
It has been conjectured (as for Wilson primes) that infinitely many Wieferich primes exist, and that the number of Wieferich primes below x is approximately log(log(x)), which is a heuristic result that follows from the plausible assumption that for a prime p, the (p − 1)th degree roots of unity modulo p^{2} are uniformly distributed in the multiplicative group of integers modulo p^{2}.^{[28]}
The following theorem connecting Wieferich primes and Fermat's Last Theorem was proven by Wieferich in 1909:^{[10]}
The above case (where p does not divide any of x, y or z) is commonly known as the first case of Fermat's Last Theorem (FLTI)^{[29]}^{[30]} and FLTI is said to fail for a prime p, if solutions to the Fermat equation exist for that p, otherwise FLTI holds for p.^{[31]} In 1910, Mirimanoff expanded^{[32]} the theorem by showing that, if the preconditions of the theorem hold true for some prime p, then p^{2} must also divide 3^{p − 1} − 1. Granville and Monagan further proved that p^{2} must actually divide m^{p − 1} − 1 for every prime m ≤ 89.^{[33]} Suzuki extended the proof to all primes m ≤ 113.^{[34]}
Let H_{p} be a set of pairs of integers with 1 as their greatest common divisor, p being prime to x, y and x + y, (x + y)^{p−1} ≡ 1 (mod p^{2}), (x + ξy) being the pth power of an ideal of K with ξ defined as cos 2π/p + i sin 2π/p. K = Q(ξ) is the field extension obtained by adjoining all polynomials in the algebraic number ξ to the field of rational numbers (such an extension is known as a number field or in this particular case, where ξ is a root of unity, a cyclotomic number field).^{[33]}^{: 332 } From uniqueness of factorization of ideals in Q(ξ) it follows that if the first case of Fermat's last theorem has solutions x, y, z then p divides x+y+z and (x, y), (y, z) and (z, x) are elements of H_{p}.^{[33]}^{: 333 } Granville and Monagan showed that (1, 1) ∈ H_{p} if and only if p is a Wieferich prime.^{[33]}^{: 333 }
A nonWieferich prime is a prime p satisfying 2^{p − 1} ≢ 1 (mod p^{2}). J. H. Silverman showed in 1988 that if the abc conjecture holds, then there exist infinitely many nonWieferich primes.^{[35]} More precisely he showed that the abc conjecture implies the existence of a constant only depending on α such that the number of nonWieferich primes to base α with p less than or equal to a variable X is greater than log(X) as X goes to infinity.^{[36]}^{: 227 } Numerical evidence suggests that very few of the prime numbers in a given interval are Wieferich primes. The set of Wieferich primes and the set of nonWieferich primes, sometimes denoted by W_{2} and W_{2}^{c} respectively,^{[37]} are complementary sets, so if one of them is shown to be finite, the other one would necessarily have to be infinite, because both are proper subsets of the set of prime numbers. It was later shown that the existence of infinitely many nonWieferich primes already follows from a weaker version of the abc conjecture, called the ABC(k, ε) conjecture.^{[38]} Additionally, the existence of infinitely many nonWieferich primes would also follow if there exist infinitely many squarefree Mersenne numbers^{[39]} as well as if there exists a real number ξ such that the set {n ∈ N : λ(2^{n} − 1) < 2 − ξ} is of density one, where the index of composition λ(n) of an integer n is defined as and , meaning gives the product of all prime factors of n.^{[37]}^{: 4 }
It is known that the nth Mersenne number M_{n} = 2^{n} − 1 is prime only if n is prime. Fermat's little theorem implies that if p > 2 is prime, then M_{p−1} (= 2^{p − 1} − 1) is always divisible by p. Since Mersenne numbers of prime indices M_{p} and M_{q} are coprime,
Thus, a Mersenne prime cannot also be a Wieferich prime. A notable open problem is to determine whether or not all Mersenne numbers of prime index are squarefree. If q is prime and the Mersenne number M_{q} is not squarefree, that is, there exists a prime p for which p^{2} divides M_{q}, then p is a Wieferich prime. Therefore, if there are only finitely many Wieferich primes, then there will be at most finitely many Mersenne numbers with prime index that are not squarefree. Rotkiewicz showed a related result: if there are infinitely many squarefree Mersenne numbers, then there are infinitely many nonWieferich primes.^{[41]}
Similarly, if p is prime and p^{2} divides some Fermat number F_{n} = 2^{2n} + 1, then p must be a Wieferich prime.^{[42]}
In fact, there exists a natural number n and a prime p that p^{2} divides (where is the nth cyclotomic polynomial) if and only if p is a Wieferich prime. For example, 1093^{2} divides , 3511^{2} divides . Mersenne and Fermat numbers are just special situations of . Thus, if 1093 and 3511 are only two Wieferich primes, then all are squarefree except and (In fact, when there exists a prime p which p^{2} divides some , then it is a Wieferich prime); and clearly, if is a prime, then it cannot be Wieferich prime. (Any odd prime p divides only one and n divides p − 1, and if and only if the period length of 1/p in binary is n, then p divides . Besides, if and only if p is a Wieferich prime, then the period length of 1/p and 1/p^{2} are the same (in binary). Otherwise, this is p times than that.)
For the primes 1093 and 3511, it was shown that neither of them is a divisor of any Mersenne number with prime index nor a divisor of any Fermat number, because 364 and 1755 are neither prime nor powers of 2.^{[43]}
Scott and Styer showed that the equation p^{x} – 2^{y} = d has at most one solution in positive integers (x, y), unless when p^{4}  2^{ordp 2} – 1 if p ≢ 65 (mod 192) or unconditionally when p^{2}  2^{ordp 2} – 1, where ord_{p} 2 denotes the multiplicative order of 2 modulo p.^{[44]}^{: 215, 217–218 } They also showed that a solution to the equation ±a^{x1} ± 2^{y1} = ±a^{x2} ± 2^{y2} = c must be from a specific set of equations but that this does not hold, if a is a Wieferich prime greater than 1.25 x 10^{15}.^{[45]}^{: 258 }
Johnson observed^{[46]} that the two known Wieferich primes are one greater than numbers with periodic binary expansions (1092 = 010001000100_{2}=444_{16}; 3510 = 110110110110_{2}=6666_{8}). The Wieferich@Home project searched for Wieferich primes by testing numbers that are one greater than a number with a periodic binary expansion, but up to a "bit pseudolength" of 3500 of the tested binary numbers generated by combination of bit strings with a bit length of up to 24 it has not found a new Wieferich prime.^{[47]}
It has been noted (sequence A239875 in the OEIS) that the known Wieferich primes are one greater than mutually friendly numbers (the shared abundancy index being 112/39).
It was observed that the two known Wieferich primes are the square factors of all nonsquare free base2 Fermat pseudoprimes up to 25×10^{9}.^{[48]} Later computations showed that the only repeated factors of the pseudoprimes up to 10^{12} are 1093 and 3511.^{[49]} In addition, the following connection exists:
For all primes p up to 100000, L(p^{n+1}) = L(p^{n}) only in two cases: L(1093^{2}) = L(1093) = 364 and L(3511^{2}) = L(3511) = 1755, where L(m) is the number of vertices in the cycle of 1 in the doubling diagram modulo m. Here the doubling diagram represents the directed graph with the nonnegative integers less than m as vertices and with directed edges going from each vertex x to vertex 2x reduced modulo m.^{[50]}^{: 74 } It was shown, that for all odd prime numbers either L(p^{n+1}) = p · L(p^{n}) or L(p^{n+1}) = L(p^{n}).^{[50]}^{: 75 }
It was shown that and if and only if 2^{p − 1} ≢ 1 (mod p^{2}) where p is an odd prime and is the fundamental discriminant of the imaginary quadratic field . Furthermore, the following was shown: Let p be a Wieferich prime. If p ≡ 3 (mod 4), let be the fundamental discriminant of the imaginary quadratic field and if p ≡ 1 (mod 4), let be the fundamental discriminant of the imaginary quadratic field . Then and (χ and λ in this context denote Iwasawa invariants).^{[51]}^{: 27 }
Furthermore, the following result was obtained: Let q be an odd prime number, k and p are primes such that p = 2k + 1, k ≡ 3 (mod 4), p ≡ −1 (mod q), p ≢ −1 (mod q^{3}) and the order of q modulo k is . Assume that q divides h^{+}, the class number of the real cyclotomic field , the cyclotomic field obtained by adjoining the sum of a pth root of unity and its reciprocal to the field of rational numbers. Then q is a Wieferich prime.^{[52]}^{: 55 } This also holds if the conditions p ≡ −1 (mod q) and p ≢ −1 (mod q^{3}) are replaced by p ≡ −3 (mod q) and p ≢ −3 (mod q^{3}) as well as when the condition p ≡ −1 (mod q) is replaced by p ≡ −5 (mod q) (in which case q is a Wall–Sun–Sun prime) and the incongruence condition replaced by p ≢ −5 (mod q^{3}).^{[53]}^{: 376 }
A prime p satisfying the congruence 2^{(p−1)/2} ≡ ±1 + Ap (mod p^{2}) with small A is commonly called a nearWieferich prime (sequence A195988 in the OEIS).^{[28]}^{[54]} NearWieferich primes with A = 0 represent Wieferich primes. Recent searches, in addition to their primary search for Wieferich primes, also tried to find nearWieferich primes.^{[23]}^{[55]} The following table lists all nearWieferich primes with A ≤ 10 in the interval [1×10^{9}, 3×10^{15}].^{[56]} This search bound was reached in 2006 in a search effort by P. Carlisle, R. Crandall and M. Rodenkirch.^{[22]}^{[57]} Bigger entries are by PrimeGrid.
p  1 or −1  A 

3520624567  +1  −6 
46262476201  +1  +5 
47004625957  −1  +1 
58481216789  −1  +5 
76843523891  −1  +1 
1180032105761  +1  −6 
12456646902457  +1  +2 
134257821895921  +1  +10 
339258218134349  −1  +2 
2276306935816523  −1  −3 
82687771042557349  1  10 
3156824277937156367  +1  +7 
The sign +1 or 1 above can be easily predicted by Euler's criterion (and the second supplement to the law of quadratic reciprocity).
Dorais and Klyve^{[23]} used a different definition of a nearWieferich prime, defining it as a prime p with small value of where is the Fermat quotient of 2 with respect to p modulo p (the modulo operation here gives the residue with the smallest absolute value). The following table lists all primes p ≤ 6.7 × 10^{15} with .
p  

1093  0  0 
3511  0  0 
2276306935816523  +6  0.264 
3167939147662997  −17  0.537 
3723113065138349  −36  0.967 
5131427559624857  −36  0.702 
5294488110626977  −31  0.586 
6517506365514181  +58  0.890 
The two notions of nearness are related as follows. If , then by squaring, clearly . So if A had been chosen with small, then clearly is also (quite) small, and an even number. However, when is odd above, the related A from before the last squaring was not "small". For example, with , we have which reads extremely nonnear, but after squaring this is which is a nearWieferich by the second definition.
Main article: Fermat quotient 
A Wieferich prime base a is a prime p that satisfies
Such a prime cannot divide a, since then it would also divide 1.
It's a conjecture that for every natural number a, there are infinitely many Wieferich primes in base a.
Bolyai showed that if p and q are primes, a is a positive integer not divisible by p and q such that a^{p−1} ≡ 1 (mod q), a^{q−1} ≡ 1 (mod p), then a^{pq−1} ≡ 1 (mod pq). Setting p = q leads to a^{p2−1} ≡ 1 (mod p^{2}).^{[58]}^{: 284 } It was shown that a^{p2−1} ≡ 1 (mod p^{2}) if and only if a^{p−1} ≡ 1 (mod p^{2}).^{[58]}^{: 285–286 }
Known solutions of a^{p−1} ≡ 1 (mod p^{2}) for small values of a are:^{[59]} (checked up to 5 × 10^{13})
a  primes p such that a^{p − 1} = 1 (mod p^{2})  OEIS sequence 

1  2, 3, 5, 7, 11, 13, 17, 19, 23, 29, ... (All primes)  A000040 
2  1093, 3511, ...  A001220 
3  11, 1006003, ...  A014127 
4  1093, 3511, ...  
5  2, 20771, 40487, 53471161, 1645333507, 6692367337, 188748146801, ...  A123692 
6  66161, 534851, 3152573, ...  A212583 
7  5, 491531, ...  A123693 
8  3, 1093, 3511, ...  
9  2, 11, 1006003, ...  
10  3, 487, 56598313, ...  A045616 
11  71, ...  
12  2693, 123653, ...  A111027 
13  2, 863, 1747591, ...  A128667 
14  29, 353, 7596952219, ...  A234810 
15  29131, 119327070011, ...  A242741 
16  1093, 3511, ...  
17  2, 3, 46021, 48947, 478225523351, ...  A128668 
18  5, 7, 37, 331, 33923, 1284043, ...  A244260 
19  3, 7, 13, 43, 137, 63061489, ...  A090968 
20  281, 46457, 9377747, 122959073, ...  A242982 
21  2, ...  
22  13, 673, 1595813, 492366587, 9809862296159, ...  A298951 
23  13, 2481757, 13703077, 15546404183, 2549536629329, ...  A128669 
24  5, 25633, ...  
25  2, 20771, 40487, 53471161, 1645333507, 6692367337, 188748146801, ...  
26  3, 5, 71, 486999673, 6695256707, ...  A306255 
27  11, 1006003, ...  
28  3, 19, 23, ...  
29  2, ...  
30  7, 160541, 94727075783, ...  A306256 
31  7, 79, 6451, 2806861, ...  A331424 
32  5, 1093, 3511, ...  
33  2, 233, 47441, 9639595369, ...  
34  46145917691, ...  
35  3, 1613, 3571, ...  
36  66161, 534851, 3152573, ...  
37  2, 3, 77867, 76407520781, ...  A331426 
38  17, 127, ...  
39  8039, ...  
40  11, 17, 307, 66431, 7036306088681, ...  
41  2, 29, 1025273, 138200401, ...  A331427 
42  23, 719867822369, ...  
43  5, 103, 13368932516573, ...  
44  3, 229, 5851, ...  
45  2, 1283, 131759, 157635607, ...  
46  3, 829, ...  
47  ...  
48  7, 257, ...  
49  2, 5, 491531, ...  
50  7, ... 
For more information, see^{[60]}^{[61]}^{[62]} and.^{[63]} (Note that the solutions to a = b^{k} is the union of the prime divisors of k which does not divide b and the solutions to a = b)
The smallest solutions of n^{p−1} ≡ 1 (mod p^{2}) are
There are no known solutions of n^{p−1} ≡ 1 (mod p^{2}) for n = 47, 72, 186, 187, 200, 203, 222, 231, 304, 311, 335, 355, 435, 454, 546, 554, 610, 639, 662, 760, 772, 798, 808, 812, 858, 860, 871, 983, 986, 1002, 1023, 1130, 1136, 1138, ....
It is a conjecture that there are infinitely many solutions of a^{p−1} ≡ 1 (mod p^{2}) for every natural number a.
The bases b < p^{2} which p is a Wieferich prime are (for b > p^{2}, the solutions are just shifted by k·p^{2} for k > 0), and there are p − 1 solutions < p^{2} of p and the set of the solutions congruent to p are {1, 2, 3, ..., p − 1}) (sequence A143548 in the OEIS)
p  values of b < p^{2} 

2  1 
3  1, 8 
5  1, 7, 18, 24 
7  1, 18, 19, 30, 31, 48 
11  1, 3, 9, 27, 40, 81, 94, 112, 118, 120 
13  1, 19, 22, 23, 70, 80, 89, 99, 146, 147, 150, 168 
17  1, 38, 40, 65, 75, 110, 131, 134, 155, 158, 179, 214, 224, 249, 251, 288 
19  1, 28, 54, 62, 68, 69, 99, 116, 127, 234, 245, 262, 292, 293, 299, 307, 333, 360 
23  1, 28, 42, 63, 118, 130, 170, 177, 195, 255, 263, 266, 274, 334, 352, 359, 399, 411, 466, 487, 501, 528 
29  1, 14, 41, 60, 63, 137, 190, 196, 221, 236, 267, 270, 374, 416, 425, 467, 571, 574, 605, 620, 645, 651, 704, 778, 781, 800, 827, 840 
The least base b > 1 which prime(n) is a Wieferich prime are
We can also consider the formula , (because of the generalized Fermat little theorem, is true for all prime p and all natural number a such that both a and a + 1 are not divisible by p). It's a conjecture that for every natural number a, there are infinitely many primes such that .
Known solutions for small a are: (checked up to 4 × 10^{11}) ^{[64]}
primes such that  

1  1093, 3511, ... 
2  23, 3842760169, 41975417117, ... 
3  5, 250829, ... 
4  3, 67, ... 
5  3457, 893122907, ... 
6  72673, 1108905403, 2375385997, ... 
7  13, 819381943, ... 
8  67, 139, 499, 26325777341, ... 
9  67, 887, 9257, 83449, 111539, 31832131, ... 
10  ... 
11  107, 4637, 239357, ... 
12  5, 11, 51563, 363901, 224189011, ... 
13  3, ... 
14  11, 5749, 17733170113, 140328785783, ... 
15  292381, ... 
16  4157, ... 
17  751, 46070159, ... 
18  7, 142671309349, ... 
19  17, 269, ... 
20  29, 162703, ... 
21  5, 2711, 104651, 112922981, 331325567, 13315963127, ... 
22  3, 7, 13, 94447, 1198427, 23536243, ... 
23  43, 179, 1637, 69073, ... 
24  7, 353, 402153391, ... 
25  43, 5399, 21107, 35879, ... 
26  7, 131, 653, 5237, 97003, ... 
27  2437, 1704732131, ... 
28  5, 617, 677, 2273, 16243697, ... 
29  73, 101, 6217, ... 
30  7, 11, 23, 3301, 48589, 549667, ... 
31  3, 41, 416797, ... 
32  95989, 2276682269, ... 
33  139, 1341678275933, ... 
34  83, 139, ... 
35  ... 
36  107, 137, 613, 2423, 74304856177, ... 
37  5, ... 
38  167, 2039, ... 
39  659, 9413, ... 
40  3, 23, 21029249, ... 
41  31, 71, 1934399021, 474528373843, ... 
42  4639, 1672609, ... 
43  31, 4962186419, ... 
44  36677, 17786501, ... 
45  241, 26120375473, ... 
46  5, 13877, ... 
47  13, 311, 797, 906165497, ... 
48  ... 
49  3, 13, 2141, 281833, 1703287, 4805298913, ... 
50  2953, 22409, 99241, 5427425917, ... 
Main article: Wieferich pair 
A Wieferich pair is a pair of primes p and q that satisfy
so that a Wieferich prime p ≡ 1 (mod 4) will form such a pair (p, 2): the only known instance in this case is p = 1093. There are only 7 known Wieferich pairs.^{[65]}
Start with a(1) any natural number (>1), a(n) = the smallest prime p such that (a(n − 1))^{p − 1} = 1 (mod p^{2}) but p^{2} does not divide a(n − 1) − 1 or a(n − 1) + 1. (If p^{2} divides a(n − 1) − 1 or a(n − 1) + 1, then the solution is a trivial solution) It is a conjecture that every natural number k = a(1) > 1 makes this sequence become periodic, for example, let a(1) = 2:
Let a(1) = 83:
Let a(1) = 59 (a longer sequence):
However, there are many values of a(1) with unknown status, for example, let a(1) = 3:
Let a(1) = 14:
Let a(1) = 39 (a longer sequence):
It is unknown that values for a(1) > 1 exist such that the resulting sequence does not eventually become periodic.
When a(n − 1)=k, a(n) will be (start with k = 2): 1093, 11, 1093, 20771, 66161, 5, 1093, 11, 487, 71, 2693, 863, 29, 29131, 1093, 46021, 5, 7, 281, ?, 13, 13, 25633, 20771, 71, 11, 19, ?, 7, 7, 5, 233, 46145917691, 1613, 66161, 77867, 17, 8039, 11, 29, 23, 5, 229, 1283, 829, ?, 257, 491531, ?, ... (For k = 21, 29, 47, 50, even the next value is unknown)
A Wieferich number is an odd natural number n satisfying the congruence 2^{φ(n)} ≡ 1 (mod n^{2}), where φ denotes the Euler's totient function (according to Euler's theorem, 2^{φ(n)} ≡ 1 (mod n) for every odd natural number n). If Wieferich number n is prime, then it is a Wieferich prime. The first few Wieferich numbers are:
It can be shown that if there are only finitely many Wieferich primes, then there are only finitely many Wieferich numbers. In particular, if the only Wieferich primes are 1093 and 3511, then there exist exactly 104 Wieferich numbers, which matches the number of Wieferich numbers currently known.^{[2]}
More generally, a natural number n is a Wieferich number to base a, if a^{φ(n)} ≡ 1 (mod n^{2}).^{[66]}^{: 31 }
Another definition specifies a Wieferich number as odd natural number n such that n and are not coprime, where m is the multiplicative order of 2 modulo n. The first of these numbers are:^{[67]}
As above, if Wieferich number q is prime, then it is a Wieferich prime.
A weak Wieferich prime to base a is a prime p satisfies the condition
Every Wieferich prime to base a is also a weak Wieferich prime to base a. If the base a is squarefree, then a prime p is a weak Wieferich prime to base a if and only if p is a Wieferich prime to base a.
Smallest weak Wieferich prime to base n are (start with n = 0)
For integer n ≥2, a Wieferich prime to base a with order n is a prime p satisfies the condition
Clearly, a Wieferich prime to base a with order n is also a Wieferich prime to base a with order m for all 2 ≤ m ≤ n, and Wieferich prime to base a with order 2 is equivalent to Wieferich prime to base a, so we can only consider the n ≥ 3 case. However, there are no known Wieferich prime to base 2 with order 3. The first base with known Wieferich prime with order 3 is 9, where 2 is a Wieferich prime to base 9 with order 3. Besides, both 5 and 113 are Wieferich prime to base 68 with order 3.
Let P and Q be integers. The Lucas sequence of the first kind associated with the pair (P, Q) is defined by
for all . A Lucas–Wieferich prime associated with (P, Q) is a prime p such that U_{p−ε}(P, Q) ≡ 0 (mod p^{2}), where ε equals the Legendre symbol . All Wieferich primes are Lucas–Wieferich primes associated with the pair (3, 2).^{[3]}^{: 2088 }
Let Q = −1. For every natural number P, the Lucas–Wieferich primes associated with (P, −1) are called PFibonacci–Wieferich primes or PWall–Sun–Sun primes. If P = 1, they are called Fibonacci–Wieferich primes. If P = 2, they are called Pell–Wieferich primes.
For example, 241 is a Lucas–Wieferich prime associated with (3, −1), so it is a 3Fibonacci–Wieferich prime or 3Wall–Sun–Sun prime. In fact, 3 is a PFibonacci–Wieferich prime if and only if P congruent to 0, 4, or 5 (mod 9),^{[citation needed]} which is analogous to the statement for traditional Wieferich primes that 3 is a basen Wieferich prime if and only if n congruent to 1 or 8 (mod 9).
Let K be a global field, i.e. a number field or a function field in one variable over a finite field and let E be an elliptic curve. If v is a nonarchimedean place of norm q_{v} of K and a ∈ K, with v(a) = 0 then v(a^{qv − 1} − 1) ≥ 1. v is called a Wieferich place for base a, if v(a^{qv − 1} − 1) > 1, an elliptic Wieferich place for base P ∈ E, if N_{v}P ∈ E_{2} and a strong elliptic Wieferich place for base P ∈ E if n_{v}P ∈ E_{2}, where n_{v} is the order of P modulo v and N_{v} gives the number of rational points (over the residue field of v) of the reduction of E at v.^{[68]}^{: 206 }