81220721551 has 12 divisors (see below), whose sum is σ = 86295828528. Its totient is φ = 76428522752.

The previous prime is 81220721539. The next prime is 81220721563. The reversal of 81220721551 is 15512702218.

It is an interprime number because it is at equal distance from previous prime (81220721539) and next prime (81220721563).

It is not a de Polignac number, because 81220721551 - 2¹⁹ = 81220197263 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81220721581) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1689195 + ... + 1736611.

It is an arithmetic number, because the mean of its divisors is an integer number (7191319044).

Almost surely, 2^81220721551 is an apocalyptic number.

81220721551 is a deficient number, since it is larger than the sum of its proper divisors (5075106977).

81220721551 is a wasteful number, since it uses less digits than its factorization.

81220721551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 53378 (or 53361 counting only the distinct ones).

The product of its (nonzero) digits is 11200, while the sum is 34.

The spelling of 81220721551 in words is "eighty-one billion, two hundred twenty million, seven hundred twenty-one thousand, five hundred fifty-one".