81661321864 has 8 divisors (see below), whose sum is σ = 153114978510. Its totient is φ = 40830660928.

The previous prime is 81661321813. The next prime is 81661321877. The reversal of 81661321864 is 46812316618.

It can be written as a sum of positive squares in only one way, i.e., 78993599364 + 2667722500 = 281058^2 + 51650^2 .

It is a tau number, because it is divible by the number of its divisors (8).

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5103832609 + ... + 5103832624.

Almost surely, 2^81661321864 is an apocalyptic number.

It is an amenable number.

81661321864 is a deficient number, since it is larger than the sum of its proper divisors (71453656646).

81661321864 is a wasteful number, since it uses less digits than its factorization.

81661321864 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10207665239 (or 10207665235 counting only the distinct ones).

The product of its digits is 331776, while the sum is 46.

The spelling of 81661321864 in words is "eighty-one billion, six hundred sixty-one million, three hundred twenty-one thousand, eight hundred sixty-four".