21705649251 has 48 divisors (see below), whose sum is σ = 34477417632. Its totient is φ = 12065264640.

The previous prime is 21705649247. The next prime is 21705649291. The reversal of 21705649251 is 15294650712.

It is not a de Polignac number, because 21705649251 - 2² = 21705649247 is a prime.

It is a super-2 number, since 2×21705649251² (a number of 21 digits) contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21705649241) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 726235 + ... + 755531.

It is an arithmetic number, because the mean of its divisors is an integer number (718279534).

Almost surely, 2^21705649251 is an apocalyptic number.

21705649251 is a deficient number, since it is larger than the sum of its proper divisors (12771768381).

21705649251 is a wasteful number, since it uses less digits than its factorization.

21705649251 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 29492 (or 29481 counting only the distinct ones).

The product of its (nonzero) digits is 151200, while the sum is 42.

The spelling of 21705649251 in words is "twenty-one billion, seven hundred five million, six hundred forty-nine thousand, two hundred fifty-one".