21255581 has 2 divisors, whose sum is σ = 21255582. Its totient is φ = 21255580.

The previous prime is 21255569. The next prime is 21255587. The reversal of 21255581 is 18555212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21252100 + 3481 = 4610^2 + 59^2 .

It is a cyclic number.

It is not a de Polignac number, because 21255581 - 2¹⁸ = 20993437 is a prime.

It is a super-2 number, since 2×21255581² = 903599447295122, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a plaindrome in base 16.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21255587) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10627790 + 10627791.

It is an arithmetic number, because the mean of its divisors is an integer number (10627791).

Almost surely, 2^21255581 is an apocalyptic number.

It is an amenable number.

21255581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21255581 is an equidigital number, since it uses as much as digits as its factorization.

21255581 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4000, while the sum is 29.

The square root of 21255581 is about 4610.3775333480. The cubic root of 21255581 is about 277.0071592115.

The spelling of 21255581 in words is "twenty-one million, two hundred fifty-five thousand, five hundred eighty-one".