21555917 has 2 divisors, whose sum is σ = 21555918. Its totient is φ = 21555916.

The previous prime is 21555893. The next prime is 21555923. The reversal of 21555917 is 71955512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 14485636 + 7070281 = 3806^2 + 2659^2 .

It is a cyclic number.

It is not a de Polignac number, because 21555917 - 2²⁴ = 4778701 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21555967) 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, 10777958 + 10777959.

It is an arithmetic number, because the mean of its divisors is an integer number (10777959).

Almost surely, 2^21555917 is an apocalyptic number.

It is an amenable number.

21555917 is a deficient number, since it is larger than the sum of its proper divisors (1).

21555917 is an equidigital number, since it uses as much as digits as its factorization.

21555917 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 15750, while the sum is 35.

The square root of 21555917 is about 4642.8350175297. The cubic root of 21555917 is about 278.3057425011.

The spelling of 21555917 in words is "twenty-one million, five hundred fifty-five thousand, nine hundred seventeen".