21509731 has 2 divisors, whose sum is σ = 21509732. Its totient is φ = 21509730.

The previous prime is 21509723. The next prime is 21509737. The reversal of 21509731 is 13790512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21509731 - 2³ = 21509723 is a prime.

It is a super-2 number, since 2×21509731² = 925337055384722, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21509731.

It is not a weakly prime, because it can be changed into another prime (21509737) 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 as a sum of consecutive naturals, namely, 10754865 + 10754866.

It is an arithmetic number, because the mean of its divisors is an integer number (10754866).

Almost surely, 2^21509731 is an apocalyptic number.

21509731 is a deficient number, since it is larger than the sum of its proper divisors (1).

21509731 is an equidigital number, since it uses as much as digits as its factorization.

21509731 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1890, while the sum is 28.

The square root of 21509731 is about 4637.8584497589. The cubic root of 21509731 is about 278.1068331525.

It can be divided in two parts, 2150 and 9731, that added together give a square (11881 = 109²).

The spelling of 21509731 in words is "twenty-one million, five hundred nine thousand, seven hundred thirty-one".