21539997 has 12 divisors (see below), whose sum is σ = 31160220. Its totient is φ = 14338368.

The previous prime is 21539993. The next prime is 21540017. The reversal of 21539997 is 79993512.

It is a happy number.

21539997 is a `hidden beast` number, since 21 + 539 + 9 + 97 = 666.

It can be written as a sum of positive squares in 2 ways, for example, as 19660356 + 1879641 = 4434^2 + 1371^2 .

It is not a de Polignac number, because 21539997 - 2² = 21539993 is a prime.

It is a hoax number, since the sum of its digits (45) coincides with the sum of the digits of its distinct prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21539993) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 6529 + ... + 9257.

It is an arithmetic number, because the mean of its divisors is an integer number (2596685).

Almost surely, 2^21539997 is an apocalyptic number.

It is an amenable number.

21539997 is a deficient number, since it is larger than the sum of its proper divisors (9620223).

21539997 is a wasteful number, since it uses less digits than its factorization.

21539997 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3612 (or 3609 counting only the distinct ones).

The product of its digits is 153090, while the sum is 45.

The square root of 21539997 is about 4641.1202311511. The cubic root of 21539997 is about 278.2372119192.

It can be divided in two parts, 215399 and 97, that added together give a triangular number (215496 = T₆₅₆).

The spelling of 21539997 in words is "twenty-one million, five hundred thirty-nine thousand, nine hundred ninety-seven".