21287731 has 16 divisors (see below), whose sum is σ = 22763520. Its totient is φ = 19877760.

The previous prime is 21287729. The next prime is 21287737. The reversal of 21287731 is 13778212.

It is a cyclic number.

It is not a de Polignac number, because 21287731 - 2¹ = 21287729 is a prime.

It is a super-2 number, since 2×21287731² = 906334982256722, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (31).

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21287693 and 21287702.

It is not an unprimeable number, because it can be changed into a prime (21287737) 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, 188331 + ... + 188443.

It is an arithmetic number, because the mean of its divisors is an integer number (1422720).

Almost surely, 2^21287731 is an apocalyptic number.

21287731 is a deficient number, since it is larger than the sum of its proper divisors (1475789).

21287731 is a wasteful number, since it uses less digits than its factorization.

21287731 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 306.

The product of its digits is 4704, while the sum is 31.

The square root of 21287731 is about 4613.8629151721. The cubic root of 21287731 is about 277.1467506880.

The spelling of 21287731 in words is "twenty-one million, two hundred eighty-seven thousand, seven hundred thirty-one".