25  

Cardinal  twentyfive 
Ordinal  25th (twentyfifth) 
Factorization  5^{2} 
Divisors  1, 5, 25 
Roman numeral  XXV 
Binary  11001_{2} 
Octal  31_{8} 
Duodecimal  21_{12} 
Hexadecimal  19_{16} 
25 (twentyfive) is the natural number following 24 and preceding 26.
Contents 
It is a square number, being 5² = 5 × 5. It is the smallest square that is also a sum of two (nonzero) squares: 25 = 3² + 4². Hence it often appears in illustrations of the Pythagorean theorem.
25 is a centered octagonal number and an automorphic number.
25 per cent means one quarter.
25 has an aliquot sum of 6 and is the first number to have an aliquot sequence that does not culminate in 0 through a prime. Twentyfive is the aliquot sum of three integers; 95, 119, and 143. Twentyfive is the second composite member of the 6aliquot 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^{n} = 7 mod n.
25 is the smallest aspiring number — a composite nonsociable number whose aliquot sequence does not terminate.
According to the Shapiro inequality 25 is the least odd integer n such that there exist such that
where x_{n + 1} = x_{1},x_{n + 2} = x_{2}.
Within base 10 one can readily test for divisibility by 25 by seeing if the last two digits of the number match 25, 50, 75 or 00.
25 and 49 are the only perfect squares in the following list:
13,25,37,49,511,613,715,817,919,1021,1123,1225,1327,1429...etc
The formula in this list can be described as 10^{n} * Z + (2Z + 1) where n clearly depends on the number of digits in Z and in 2Z+1.
Twenty five is:
25 A.D., 25 B.C., 1925, 2025, etc.
Twentyfive is a number. It comes between twentyfour and twentysix, and is an odd number. It is also the 5th square number, after 1, 4, 9, and 16.
