21581973 has 24 divisors (see below), whose sum is σ = 36166208. Its totient is φ = 12146112.

The previous prime is 21581969. The next prime is 21582013. The reversal of 21581973 is 37918512.

21581973 is a `hidden beast` number, since 2 + 1 + 581 + 9 + 73 = 666.

It is not a de Polignac number, because 21581973 - 2² = 21581969 is a prime.

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

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21581873) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 1665 + ... + 6777.

Almost surely, 2^21581973 is an apocalyptic number.

It is an amenable number.

21581973 is a deficient number, since it is larger than the sum of its proper divisors (14584235).

21581973 is a wasteful number, since it uses less digits than its factorization.

21581973 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5193 (or 5190 counting only the distinct ones).

The product of its digits is 15120, while the sum is 36.

The square root of 21581973 is about 4645.6402142224. The cubic root of 21581973 is about 278.4178325998.

The spelling of 21581973 in words is "twenty-one million, five hundred eighty-one thousand, nine hundred seventy-three".