21356661 has 16 divisors (see below), whose sum is σ = 29154816. Its totient is φ = 13902336.

The previous prime is 21356653. The next prime is 21356689. The reversal of 21356661 is 16665312.

It is not a de Polignac number, because 21356661 - 2³ = 21356653 is a prime.

It is a super-2 number, since 2×21356661² = 912213938137842, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21356611) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 24316 + ... + 25178.

It is an arithmetic number, because the mean of its divisors is an integer number (1822176).

Almost surely, 2^21356661 is an apocalyptic number.

It is an amenable number.

21356661 is a deficient number, since it is larger than the sum of its proper divisors (7798155).

21356661 is a wasteful number, since it uses less digits than its factorization.

21356661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1052.

The product of its digits is 6480, while the sum is 30.

The square root of 21356661 is about 4621.3267575449. The cubic root of 21356661 is about 277.4455635449.

The spelling of 21356661 in words is "twenty-one million, three hundred fifty-six thousand, six hundred sixty-one".