21591217 has 2 divisors, whose sum is σ = 21591218. Its totient is φ = 21591216.

The previous prime is 21591211. The next prime is 21591233. The reversal of 21591217 is 71219512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12645136 + 8946081 = 3556^2 + 2991^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21591217 is a prime.

It is not a weakly prime, because it can be changed into another prime (21591211) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10795608 + 10795609.

It is an arithmetic number, because the mean of its divisors is an integer number (10795609).

Almost surely, 2^21591217 is an apocalyptic number.

It is an amenable number.

21591217 is a deficient number, since it is larger than the sum of its proper divisors (1).

21591217 is an equidigital number, since it uses as much as digits as its factorization.

21591217 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1260, while the sum is 28.

The square root of 21591217 is about 4646.6350190218. The cubic root of 21591217 is about 278.4575776079.

The spelling of 21591217 in words is "twenty-one million, five hundred ninety-one thousand, two hundred seventeen".