81521601643 has 4 divisors (see below), whose sum is σ = 83417452888. Its totient is φ = 79625750400.

The previous prime is 81521601529. The next prime is 81521601653. The reversal of 81521601643 is 34610612518.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 34610612518 = 2 ⋅17305306259.

It is a cyclic number.

It is not a de Polignac number, because 81521601643 - 2²⁵ = 81488047211 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81521601653) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 947925558 + ... + 947925643.

It is an arithmetic number, because the mean of its divisors is an integer number (20854363222).

Almost surely, 2^81521601643 is an apocalyptic number.

81521601643 is a deficient number, since it is larger than the sum of its proper divisors (1895851245).

81521601643 is a wasteful number, since it uses less digits than its factorization.

81521601643 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1895851244.

The product of its (nonzero) digits is 34560, while the sum is 37.

The spelling of 81521601643 in words is "eighty-one billion, five hundred twenty-one million, six hundred one thousand, six hundred forty-three".