21687857 has 2 divisors, whose sum is σ = 21687858. Its totient is φ = 21687856.

The previous prime is 21687803. The next prime is 21687863. The reversal of 21687857 is 75878612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17986081 + 3701776 = 4241^2 + 1924^2 .

It is a cyclic number.

It is not a de Polignac number, because 21687857 - 2⁶ = 21687793 is a prime.

It is not a weakly prime, because it can be changed into another prime (21687877) 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, 10843928 + 10843929.

It is an arithmetic number, because the mean of its divisors is an integer number (10843929).

Almost surely, 2^21687857 is an apocalyptic number.

It is an amenable number.

21687857 is a deficient number, since it is larger than the sum of its proper divisors (1).

21687857 is an equidigital number, since it uses as much as digits as its factorization.

21687857 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 188160, while the sum is 44.

The square root of 21687857 is about 4657.0223319198. The cubic root of 21687857 is about 278.8724081825.

The spelling of 21687857 in words is "twenty-one million, six hundred eighty-seven thousand, eight hundred fifty-seven".