21751664661 has 6 divisors (see below), whose sum is σ = 31419071190. Its totient is φ = 14501109768.

The previous prime is 21751664617. The next prime is 21751664677. The reversal of 21751664661 is 16646615712.

21751664661 is a `hidden beast` number, since 2 + 17 + 516 + 64 + 6 + 61 = 666.

It can be written as a sum of positive squares in only one way, i.e., 11277165636 + 10474499025 = 106194^2 + 102345^2 .

It is not a de Polignac number, because 21751664661 - 2⁷ = 21751664533 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21751664161) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1208425806 + ... + 1208425823.

It is an arithmetic number, because the mean of its divisors is an integer number (5236511865).

Almost surely, 2^21751664661 is an apocalyptic number.

It is an amenable number.

21751664661 is a deficient number, since it is larger than the sum of its proper divisors (9667406529).

21751664661 is a wasteful number, since it uses less digits than its factorization.

21751664661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2416851635 (or 2416851632 counting only the distinct ones).

The product of its digits is 362880, while the sum is 45.

The spelling of 21751664661 in words is "twenty-one billion, seven hundred fifty-one million, six hundred sixty-four thousand, six hundred sixty-one".