21696581 has 2 divisors, whose sum is σ = 21696582. Its totient is φ = 21696580.

The previous prime is 21696551. The next prime is 21696583. The reversal of 21696581 is 18569612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21669025 + 27556 = 4655^2 + 166^2 .

It is a cyclic number.

It is not a de Polignac number, because 21696581 - 2¹⁸ = 21434437 is a prime.

It is a super-2 number, since 2×21696581² = 941483254179122, which contains 22 as substring.

Together with 21696583, it forms a pair of twin primes.

It is a Chen prime.

It is an alternating number because its digits alternate between even and odd.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21696583) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10848290 + 10848291.

It is an arithmetic number, because the mean of its divisors is an integer number (10848291).

Almost surely, 2^21696581 is an apocalyptic number.

It is an amenable number.

21696581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21696581 is an equidigital number, since it uses as much as digits as its factorization.

21696581 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 25920, while the sum is 38.

The square root of 21696581 is about 4657.9588877533. The cubic root of 21696581 is about 278.9097955670.

The spelling of 21696581 in words is "twenty-one million, six hundred ninety-six thousand, five hundred eighty-one".