1656561991 has 32 divisors (see below), whose sum is σ = 2067061248. Its totient is φ = 1295170560.

The previous prime is 1656561943. The next prime is 1656562001. The reversal of 1656561991 is 1991656561.

It is a cyclic number.

It is not a de Polignac number, because 1656561991 - 2²⁵ = 1623007559 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1656561931) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 794236 + ... + 796318.

It is an arithmetic number, because the mean of its divisors is an integer number (64595664).

Almost surely, 2^1656561991 is an apocalyptic number.

1656561991 is a deficient number, since it is larger than the sum of its proper divisors (410499257).

1656561991 is a wasteful number, since it uses less digits than its factorization.

1656561991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2311.

The product of its digits is 437400, while the sum is 49.

The square root of 1656561991 is about 40700.8844007105. The cubic root of 1656561991 is about 1183.2301592428.

The spelling of 1656561991 in words is "one billion, six hundred fifty-six million, five hundred sixty-one thousand, nine hundred ninety-one".