It is a square number, being 52 = 5 × 5. 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. 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 single-digit odd natural numbers 1, 3, 5, 7 and 9, the first five odd natural numbers.
25 is a centered octagonal number, a centered square number, and an automorphic number.
25 percent (%) is equal to 1/4.
It is the smallest base 10 Friedman number as it can be expressed by its own digits: 52.
It is also a Cullen number. 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.
According to the Shapiro inequality, 25 is the least odd integer n such that there exist x1, x2, …, xn such that
where xn + 1 = x1, xn + 2 = x2.
Within base 10 one can readily test for divisibility by 25 by seeing if the last two digits of the number match 00, 25, 50 or 75.