81556507561 has 9 divisors (see below), whose sum is σ = 81891721821. Its totient is φ = 81222663372.

The previous prime is 81556507531. The next prime is 81556507577. The reversal of 81556507561 is 16570565518.

The square root of 81556507561 is 285581.

It is a perfect power (a square), and thus also a powerful number.

It is not a de Polignac number, because 81556507561 - 2¹³ = 81556499369 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81556507531) by changing a digit.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 99096196 + ... + 99097018.

Almost surely, 2^81556507561 is an apocalyptic number.

81556507561 is the 285581-st square number.

81556507561 is the 142791-st centered octagonal number.

It is an amenable number.

81556507561 is a deficient number, since it is larger than the sum of its proper divisors (335214260).

81556507561 is an frugal number, since it uses more digits than its factorization.

81556507561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2340 (or 1170 counting only the distinct ones).

The product of its (nonzero) digits is 1260000, while the sum is 49.

Multiplying 81556507561 by its sum of digits (49), we get a square (3996268870489 = 1999067²).

The spelling of 81556507561 in words is "eighty-one billion, five hundred fifty-six million, five hundred seven thousand, five hundred sixty-one".