21561817 has 2 divisors, whose sum is σ = 21561818. Its totient is φ = 21561816.

The previous prime is 21561781. The next prime is 21561851. The reversal of 21561817 is 71816512.

Together with previous prime (21561781) it forms an Ormiston pair, because they use the same digits, order apart.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16128256 + 5433561 = 4016^2 + 2331^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21561817 is a prime.

It is not a weakly prime, because it can be changed into another prime (21561887) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10780908 + 10780909.

It is an arithmetic number, because the mean of its divisors is an integer number (10780909).

Almost surely, 2^21561817 is an apocalyptic number.

It is an amenable number.

21561817 is a deficient number, since it is larger than the sum of its proper divisors (1).

21561817 is an equidigital number, since it uses as much as digits as its factorization.

21561817 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3360, while the sum is 31.

The square root of 21561817 is about 4643.4703617015. The cubic root of 21561817 is about 278.3311315717.

The spelling of 21561817 in words is "twenty-one million, five hundred sixty-one thousand, eight hundred seventeen".