21557971 has 2 divisors, whose sum is σ = 21557972. Its totient is φ = 21557970.

The previous prime is 21557957. The next prime is 21557983. The reversal of 21557971 is 17975512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21557971 - 2⁷ = 21557843 is a prime.

It is a super-2 number, since 2×21557971² = 929492227273682, 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 21557971.

It is not a weakly prime, because it can be changed into another prime (21557941) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10778985 + 10778986.

It is an arithmetic number, because the mean of its divisors is an integer number (10778986).

Almost surely, 2^21557971 is an apocalyptic number.

21557971 is a deficient number, since it is larger than the sum of its proper divisors (1).

21557971 is an equidigital number, since it uses as much as digits as its factorization.

21557971 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 22050, while the sum is 37.

The square root of 21557971 is about 4643.0562133147. The cubic root of 21557971 is about 278.3145818659.

The spelling of 21557971 in words is "twenty-one million, five hundred fifty-seven thousand, nine hundred seventy-one".