664540781 has 2 divisors, whose sum is σ = 664540782. Its totient is φ = 664540780.

The previous prime is 664540733. The next prime is 664540813. The reversal of 664540781 is 187045466.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 535922500 + 128618281 = 23150^2 + 11341^2 .

It is a cyclic number.

It is not a de Polignac number, because 664540781 - 2⁶ = 664540717 is a prime.

It is a super-2 number, since 2×664540781² = 883228899224179922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (660540781) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 332270390 + 332270391.

It is an arithmetic number, because the mean of its divisors is an integer number (332270391).

Almost surely, 2^664540781 is an apocalyptic number.

It is an amenable number.

664540781 is a deficient number, since it is larger than the sum of its proper divisors (1).

664540781 is an equidigital number, since it uses as much as digits as its factorization.

664540781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 161280, while the sum is 41.

The square root of 664540781 is about 25778.6885042665. The cubic root of 664540781 is about 872.6509098770.

The spelling of 664540781 in words is "six hundred sixty-four million, five hundred forty thousand, seven hundred eighty-one".