691921561 has 2 divisors, whose sum is σ = 691921562. Its totient is φ = 691921560.

The previous prime is 691921543. The next prime is 691921589. The reversal of 691921561 is 165129196.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 689377536 + 2544025 = 26256^2 + 1595^2 .

It is a cyclic number.

It is not a de Polignac number, because 691921561 - 2¹¹ = 691919513 is a prime.

It is not a weakly prime, because it can be changed into another prime (691921541) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 345960780 + 345960781.

It is an arithmetic number, because the mean of its divisors is an integer number (345960781).

Almost surely, 2^691921561 is an apocalyptic number.

It is an amenable number.

691921561 is a deficient number, since it is larger than the sum of its proper divisors (1).

691921561 is an equidigital number, since it uses as much as digits as its factorization.

691921561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 29160, while the sum is 40.

The square root of 691921561 is about 26304.4019319961. The cubic root of 691921561 is about 884.4751208985.

The spelling of 691921561 in words is "six hundred ninety-one million, nine hundred twenty-one thousand, five hundred sixty-one".