21172619921 has 2 divisors, whose sum is σ = 21172619922. Its totient is φ = 21172619920.

The previous prime is 21172619891. The next prime is 21172619929. The reversal of 21172619921 is 12991627112.

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., 17885317696 + 3287302225 = 133736^2 + 57335^2 .

It is a cyclic number.

It is not a de Polignac number, because 21172619921 - 2³⁰ = 20098878097 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21172619929) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10586309960 + 10586309961.

It is an arithmetic number, because the mean of its divisors is an integer number (10586309961).

Almost surely, 2^21172619921 is an apocalyptic number.

It is an amenable number.

21172619921 is a deficient number, since it is larger than the sum of its proper divisors (1).

21172619921 is an equidigital number, since it uses as much as digits as its factorization.

21172619921 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 27216, while the sum is 41.

The spelling of 21172619921 in words is "twenty-one billion, one hundred seventy-two million, six hundred nineteen thousand, nine hundred twenty-one".