21521178421 has 2 divisors, whose sum is σ = 21521178422. Its totient is φ = 21521178420.

The previous prime is 21521178359. The next prime is 21521178433. The reversal of 21521178421 is 12487112512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19575927396 + 1945251025 = 139914^2 + 44105^2 .

It is a cyclic number.

It is not a de Polignac number, because 21521178421 - 2²³ = 21512789813 is a prime.

It is a super-2 number, since 2×21521178421² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21521178451) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10760589210 + 10760589211.

It is an arithmetic number, because the mean of its divisors is an integer number (10760589211).

Almost surely, 2^21521178421 is an apocalyptic number.

It is an amenable number.

21521178421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21521178421 is an equidigital number, since it uses as much as digits as its factorization.

21521178421 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8960, while the sum is 34.

The spelling of 21521178421 in words is "twenty-one billion, five hundred twenty-one million, one hundred seventy-eight thousand, four hundred twenty-one".