21531231517 has 2 divisors, whose sum is σ = 21531231518. Its totient is φ = 21531231516.

The previous prime is 21531231497. The next prime is 21531231529. The reversal of 21531231517 is 71513213512.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21089138841 + 442092676 = 145221^2 + 21026^2 .

It is a cyclic number.

It is not a de Polignac number, because 21531231517 - 2⁷ = 21531231389 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21531211517) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10765615758 + 10765615759.

It is an arithmetic number, because the mean of its divisors is an integer number (10765615759).

Almost surely, 2^21531231517 is an apocalyptic number.

It is an amenable number.

21531231517 is a deficient number, since it is larger than the sum of its proper divisors (1).

21531231517 is an equidigital number, since it uses as much as digits as its factorization.

21531231517 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 6300, while the sum is 31.

The spelling of 21531231517 in words is "twenty-one billion, five hundred thirty-one million, two hundred thirty-one thousand, five hundred seventeen".