691649718521 has 2 divisors, whose sum is σ = 691649718522. Its totient is φ = 691649718520.

The previous prime is 691649718457. The next prime is 691649718547. The reversal of 691649718521 is 125817946196.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 648991360000 + 42658358521 = 805600^2 + 206539^2 .

It is a cyclic number.

It is not a de Polignac number, because 691649718521 - 2⁶ = 691649718457 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (691649718581) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 345824859260 + 345824859261.

It is an arithmetic number, because the mean of its divisors is an integer number (345824859261).

Almost surely, 2^691649718521 is an apocalyptic number.

It is an amenable number.

691649718521 is a deficient number, since it is larger than the sum of its proper divisors (1).

691649718521 is an equidigital number, since it uses as much as digits as its factorization.

691649718521 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6531840, while the sum is 59.

The spelling of 691649718521 in words is "six hundred ninety-one billion, six hundred forty-nine million, seven hundred eighteen thousand, five hundred twenty-one".