21997531 has 2 divisors, whose sum is σ = 21997532. Its totient is φ = 21997530.

The previous prime is 21997517. The next prime is 21997567. The reversal of 21997531 is 13579912.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21997531 - 2¹⁵ = 21964763 is a prime.

It is a super-2 number, since 2×21997531² = 967782740191922, which contains 22 as substring.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21997331) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10998765 + 10998766.

It is an arithmetic number, because the mean of its divisors is an integer number (10998766).

Almost surely, 2^21997531 is an apocalyptic number.

21997531 is the 2098-th centered decagonal number.

21997531 is a deficient number, since it is larger than the sum of its proper divisors (1).

21997531 is an equidigital number, since it uses as much as digits as its factorization.

21997531 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 17010, while the sum is 37.

The square root of 21997531 is about 4690.1525561542. The cubic root of 21997531 is about 280.1934504990.

It can be divided in two parts, 2199 and 7531, that added together give a triangular number (9730 = T₁₃₉).

The spelling of 21997531 in words is "twenty-one million, nine hundred ninety-seven thousand, five hundred thirty-one".