21251206665 has 72 divisors (see below), whose sum is σ = 39934769472. Its totient is φ = 10449578880.

The previous prime is 21251206597. The next prime is 21251206729. The reversal of 21251206665 is 56660215212.

It is not a de Polignac number, because 21251206665 - 2¹¹ = 21251204617 is a prime.

It is a super-2 number, since 2×21251206665² (a number of 21 digits) contains 22 as substring.

It is a hoax number, since the sum of its digits (36) coincides with the sum of the digits of its distinct prime factors.

It is an unprimeable number.

It is a polite number, since it can be written in 71 ways as a sum of consecutive naturals, for example, 11733610 + ... + 11735420.

It is an arithmetic number, because the mean of its divisors is an integer number (554649576).

Almost surely, 2^21251206665 is an apocalyptic number.

It is an amenable number.

21251206665 is a deficient number, since it is larger than the sum of its proper divisors (18683562807).

21251206665 is a wasteful number, since it uses less digits than its factorization.

21251206665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3391 (or 3375 counting only the distinct ones).

The product of its (nonzero) digits is 43200, while the sum is 36.

The spelling of 21251206665 in words is "twenty-one billion, two hundred fifty-one million, two hundred six thousand, six hundred sixty-five".