21731491 has 4 divisors (see below), whose sum is σ = 23009832. Its totient is φ = 20453152.

The previous prime is 21731477. The next prime is 21731509. The reversal of 21731491 is 19413712.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21731491 - 2⁷ = 21731363 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21731441) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 639145 + ... + 639178.

It is an arithmetic number, because the mean of its divisors is an integer number (5752458).

Almost surely, 2^21731491 is an apocalyptic number.

21731491 is a deficient number, since it is larger than the sum of its proper divisors (1278341).

21731491 is a wasteful number, since it uses less digits than its factorization.

21731491 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1278340.

The product of its digits is 1512, while the sum is 28.

The square root of 21731491 is about 4661.7047311043. The cubic root of 21731491 is about 279.0593048908.

It can be divided in two parts, 21731 and 491, that added together give a palindrome (22222).

The spelling of 21731491 in words is "twenty-one million, seven hundred thirty-one thousand, four hundred ninety-one".