21957491 has 2 divisors, whose sum is σ = 21957492. Its totient is φ = 21957490.

The previous prime is 21957473. The next prime is 21957493. The reversal of 21957491 is 19475912.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21957491 is a prime.

It is a super-2 number, since 2×21957491² = 964262822030162, which contains 22 as substring.

Together with 21957493, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21957493) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10978745 + 10978746.

It is an arithmetic number, because the mean of its divisors is an integer number (10978746).

Almost surely, 2^21957491 is an apocalyptic number.

21957491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21957491 is an equidigital number, since it uses as much as digits as its factorization.

21957491 is an evil number, because the sum of its binary digits is even.

The product of its digits is 22680, while the sum is 38.

The square root of 21957491 is about 4685.8820941206. The cubic root of 21957491 is about 280.0233441421.

It can be divided in two parts, 21 and 957491, that multiplied together give a triangular number (20107311 = T₆₃₄₁).

The spelling of 21957491 in words is "twenty-one million, nine hundred fifty-seven thousand, four hundred ninety-one".