661521557 has 4 divisors (see below), whose sum is σ = 679400556. Its totient is φ = 643642560.

The previous prime is 661521551. The next prime is 661521563. The reversal of 661521557 is 755125166.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (661521551) and next prime (661521563).

It can be written as a sum of positive squares in 2 ways, for example, as 458002801 + 203518756 = 21401^2 + 14266^2 .

It is a cyclic number.

It is not a de Polignac number, because 661521557 - 2¹⁰ = 661520533 is a prime.

It is a super-2 number, since 2×661521557² = 875221540751408498, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 661521557.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661521551) 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, 8939444 + ... + 8939517.

It is an arithmetic number, because the mean of its divisors is an integer number (169850139).

Almost surely, 2^661521557 is an apocalyptic number.

It is an amenable number.

661521557 is a deficient number, since it is larger than the sum of its proper divisors (17878999).

661521557 is a wasteful number, since it uses less digits than its factorization.

661521557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17878998.

The product of its digits is 63000, while the sum is 38.

The square root of 661521557 is about 25720.0613723996. The cubic root of 661521557 is about 871.3273247839.

The spelling of 661521557 in words is "six hundred sixty-one million, five hundred twenty-one thousand, five hundred fifty-seven".