21517581721 has 2 divisors, whose sum is σ = 21517581722. Its totient is φ = 21517581720.

The previous prime is 21517581677. The next prime is 21517581737. The reversal of 21517581721 is 12718571512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21247143696 + 270438025 = 145764^2 + 16445^2 .

It is a cyclic number.

It is not a de Polignac number, because 21517581721 - 2⁹ = 21517581209 is a prime.

It is not a weakly prime, because it can be changed into another prime (21517581521) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10758790860 + 10758790861.

It is an arithmetic number, because the mean of its divisors is an integer number (10758790861).

Almost surely, 2^21517581721 is an apocalyptic number.

It is an amenable number.

21517581721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21517581721 is an equidigital number, since it uses as much as digits as its factorization.

21517581721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 39200, while the sum is 40.

The spelling of 21517581721 in words is "twenty-one billion, five hundred seventeen million, five hundred eighty-one thousand, seven hundred twenty-one".