21685721 has 2 divisors, whose sum is σ = 21685722. Its totient is φ = 21685720.

The previous prime is 21685687. The next prime is 21685733. The reversal of 21685721 is 12758612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20830096 + 855625 = 4564^2 + 925^2 .

It is a cyclic number.

It is not a de Polignac number, because 21685721 - 2¹⁴ = 21669337 is a prime.

It is not a weakly prime, because it can be changed into another prime (21685751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10842860 + 10842861.

It is an arithmetic number, because the mean of its divisors is an integer number (10842861).

Almost surely, 2^21685721 is an apocalyptic number.

It is an amenable number.

21685721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21685721 is an equidigital number, since it uses as much as digits as its factorization.

21685721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6720, while the sum is 32.

The square root of 21685721 is about 4656.7929951846. The cubic root of 21685721 is about 278.8632526595.

The spelling of 21685721 in words is "twenty-one million, six hundred eighty-five thousand, seven hundred twenty-one".