661556009 has 2 divisors, whose sum is σ = 661556010. Its totient is φ = 661556008.

The previous prime is 661555997. The next prime is 661556011. The reversal of 661556009 is 900655166.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 648211600 + 13344409 = 25460^2 + 3653^2 .

It is a cyclic number.

It is not a de Polignac number, because 661556009 - 2⁸ = 661555753 is a prime.

Together with 661556011, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (661556059) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 330778004 + 330778005.

It is an arithmetic number, because the mean of its divisors is an integer number (330778005).

Almost surely, 2^661556009 is an apocalyptic number.

It is an amenable number.

661556009 is a deficient number, since it is larger than the sum of its proper divisors (1).

661556009 is an equidigital number, since it uses as much as digits as its factorization.

661556009 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 48600, while the sum is 38.

The square root of 661556009 is about 25720.7311132479. The cubic root of 661556009 is about 871.3424507449.

The spelling of 661556009 in words is "six hundred sixty-one million, five hundred fifty-six thousand, nine".