661918993 has 4 divisors (see below), whose sum is σ = 661985460. Its totient is φ = 661852528.

The previous prime is 661918981. The next prime is 661919033. The reversal of 661918993 is 399819166.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 539772289 + 122146704 = 23233^2 + 11052^2 .

It is a cyclic number.

It is not a de Polignac number, because 661918993 - 2¹³ = 661910801 is a prime.

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 661918993.

It is not an unprimeable number, because it can be changed into a prime (661918693) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 14938 + ... + 39331.

It is an arithmetic number, because the mean of its divisors is an integer number (165496365).

Almost surely, 2^661918993 is an apocalyptic number.

It is an amenable number.

661918993 is a deficient number, since it is larger than the sum of its proper divisors (66467).

661918993 is a wasteful number, since it uses less digits than its factorization.

661918993 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 66466.

The product of its digits is 629856, while the sum is 52.

The square root of 661918993 is about 25727.7863991444. The cubic root of 661918993 is about 871.5017849687.

The spelling of 661918993 in words is "six hundred sixty-one million, nine hundred eighteen thousand, nine hundred ninety-three".