661655261 has 2 divisors, whose sum is σ = 661655262. Its totient is φ = 661655260.

The previous prime is 661655207. The next prime is 661655263. The reversal of 661655261 is 162556166.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 432889636 + 228765625 = 20806^2 + 15125^2 .

It is a cyclic number.

It is not a de Polignac number, because 661655261 - 2¹⁴ = 661638877 is a prime.

It is a Sophie Germain prime.

Together with 661655263, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (661655263) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 330827630 + 330827631.

It is an arithmetic number, because the mean of its divisors is an integer number (330827631).

Almost surely, 2^661655261 is an apocalyptic number.

It is an amenable number.

661655261 is a deficient number, since it is larger than the sum of its proper divisors (1).

661655261 is an equidigital number, since it uses as much as digits as its factorization.

661655261 is an evil number, because the sum of its binary digits is even.

The product of its digits is 64800, while the sum is 38.

The square root of 661655261 is about 25722.6604572700. The cubic root of 661655261 is about 871.3860238540.

The spelling of 661655261 in words is "six hundred sixty-one million, six hundred fifty-five thousand, two hundred sixty-one".