2.5

25 (twenty-five) is the natural number following 24 and preceding 26. It is a square number, being 5² = 5 × 5. 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 a centered octagonal number, a centered square number, and an automorphic number. 25 per cent means one quarter. 25 has an aliquot sum of 6 and number 6 is the first (or smallest) number to have an aliquot sequence that does not culminate in 0 through a prime. Twenty-five is the aliquot sum of three integers; 95, 119, and 143. Twenty-five is the second composite member of the 6-aliquot tree. It is the smallest base 10 Friedman number as it can be expressed by its own numbers: 5². It is also a Cullen number. 25 is the smallest pseudoprime satisfying the congruence 7 = 7 mod n. 25 is the smallest aspiring number — a composite non-sociable number whose aliquot sequence does not terminate. According to the Shapiro...