21391623 has 24 divisors (see below), whose sum is σ = 33930000. Its totient is φ = 12879360.

The previous prime is 21391621. The next prime is 21391649. The reversal of 21391623 is 32619312.

21391623 is a `hidden beast` number, since 2 + 1 + 39 + 1 + 623 = 666.

It is not a de Polignac number, because 21391623 - 2¹ = 21391621 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21391621) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 16503 + ... + 17751.

It is an arithmetic number, because the mean of its divisors is an integer number (1413750).

Almost surely, 2^21391623 is an apocalyptic number.

21391623 is a deficient number, since it is larger than the sum of its proper divisors (12538377).

21391623 is a wasteful number, since it uses less digits than its factorization.

21391623 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1439 (or 1436 counting only the distinct ones).

The product of its digits is 1944, while the sum is 27.

The square root of 21391623 is about 4625.1078906335. The cubic root of 21391623 is about 277.5968787627.

The spelling of 21391623 in words is "twenty-one million, three hundred ninety-one thousand, six hundred twenty-three".