In mathematics
It is a square number, being 52 = 5 × 5, and hence the third non-unitary square prime of the form p2.
It is one of two two-digit numbers whose square and higher powers of the number also ends in the same last two digits, e.g., 252 = 625; the other is 76.
Twenty five has an even aliquot sum of 6, which is itself the first even and perfect number root of an aliquot sequence; not ending in (1 and 0).
It is the smallest square that is also a sum of two (non-zero) squares: 25 = 32 + 42. Hence, it often appears in illustrations of the Pythagorean theorem.
25 is the sum of the five consecutive single-digit odd natural numbers 1, 3, 5, 7, and 9.
25 is a centered octagonal number,[1] a centered square number,[2] a centered octahedral number,[3] and an automorphic number.[4]
25 percent (%) is equal to 1/4.
It is the smallest decimal Friedman number as it can be expressed by its own digits: 52.[5]
It is also a Cullen number[6] and a vertically symmetrical number.[7] 25 is the smallest pseudoprime satisfying the congruence 7n = 7 mod n.
25 is the smallest aspiring number — a composite non-sociable number whose aliquot sequence does not terminate.[8]
According to the Shapiro inequality, 25 is the smallest odd integer n such that there exist x1, x2, ..., xn such that
where xn + 1 = x1, xn + 2 = x2.
Within decimal, one can readily test for divisibility by 25 by seeing if the last two digits of the number match 00, 25, 50, or 75.
There are 25 primes under 100.
