21671581 has 2 divisors, whose sum is σ = 21671582. Its totient is φ = 21671580.

The previous prime is 21671567. The next prime is 21671597. The reversal of 21671581 is 18517612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21483225 + 188356 = 4635^2 + 434^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21671581 is a prime.

It is a super-2 number, since 2×21671581² = 939314846079122, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21671501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10835790 + 10835791.

It is an arithmetic number, because the mean of its divisors is an integer number (10835791).

Almost surely, 2^21671581 is an apocalyptic number.

It is an amenable number.

21671581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21671581 is an equidigital number, since it uses as much as digits as its factorization.

21671581 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3360, while the sum is 31.

The square root of 21671581 is about 4655.2745354060. The cubic root of 21671581 is about 278.8026293007.

The spelling of 21671581 in words is "twenty-one million, six hundred seventy-one thousand, five hundred eighty-one".