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, …, x_n such that

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