21538251 has 32 divisors (see below), whose sum is σ = 36933120. Its totient is φ = 12150432.

The previous prime is 21538219. The next prime is 21538261. The reversal of 21538251 is 15283512.

It is a happy number.

It is not a de Polignac number, because 21538251 - 2⁵ = 21538219 is a prime.

It is a super-2 number, since 2×21538251² = 927792512278002, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21538261) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 15001 + ... + 16373.

It is an arithmetic number, because the mean of its divisors is an integer number (1154160).

Almost surely, 2^21538251 is an apocalyptic number.

21538251 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21538251 is a deficient number, since it is larger than the sum of its proper divisors (15394869).

21538251 is a wasteful number, since it uses less digits than its factorization.

21538251 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1472 (or 1466 counting only the distinct ones).

The product of its digits is 2400, while the sum is 27.

The square root of 21538251 is about 4640.9321262005. The cubic root of 21538251 is about 278.2296938851.

It can be divided in two parts, 2153 and 8251, that added together give a square (10404 = 102²).

The spelling of 21538251 in words is "twenty-one million, five hundred thirty-eight thousand, two hundred fifty-one".