21720556 has 72 divisors (see below), whose sum is σ = 47222784. Its totient is φ = 8648640.

The previous prime is 21720511. The next prime is 21720563. The reversal of 21720556 is 65502712.

21720556 = T₁ + T₂ + ... + T₅₀₆.

21720556 is a nontrivial binomial coefficient, being equal to C(508, 3).

It is an unprimeable number.

It is the 506-th tetrahedral number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 170965 + ... + 171091.

It is an arithmetic number, because the mean of its divisors is an integer number (655872).

Almost surely, 2^21720556 is an apocalyptic number.

21720556 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

It is an amenable number.

21720556 is an abundant number, since it is smaller than the sum of its proper divisors (25502228).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21720556 is a wasteful number, since it uses less digits than its factorization.

21720556 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 191 (or 176 counting only the distinct ones).

The product of its (nonzero) digits is 4200, while the sum is 28.

The square root of 21720556 is about 4660.5317293202. The cubic root of 21720556 is about 279.0124907157.

The spelling of 21720556 in words is "twenty-one million, seven hundred twenty thousand, five hundred fifty-six".