81556445521 has 2 divisors, whose sum is σ = 81556445522. Its totient is φ = 81556445520.

The previous prime is 81556445471. The next prime is 81556445543. The reversal of 81556445521 is 12554465518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 79425330625 + 2131114896 = 281825^2 + 46164^2 .

It is a cyclic number.

It is not a de Polignac number, because 81556445521 - 2²³ = 81548056913 is a prime.

It is a super-2 number, since 2×81556445521² (a number of 23 digits) contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (81556445921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40778222760 + 40778222761.

It is an arithmetic number, because the mean of its divisors is an integer number (40778222761).

Almost surely, 2^81556445521 is an apocalyptic number.

It is an amenable number.

81556445521 is a deficient number, since it is larger than the sum of its proper divisors (1).

81556445521 is an equidigital number, since it uses as much as digits as its factorization.

81556445521 is an evil number, because the sum of its binary digits is even.

The product of its digits is 960000, while the sum is 46.

The spelling of 81556445521 in words is "eighty-one billion, five hundred fifty-six million, four hundred forty-five thousand, five hundred twenty-one".