20509661 has 2 divisors, whose sum is σ = 20509662. Its totient is φ = 20509660.

The previous prime is 20509649. The next prime is 20509663. The reversal of 20509661 is 16690502.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17724100 + 2785561 = 4210^2 + 1669^2 .

It is a cyclic number.

It is not a de Polignac number, because 20509661 - 2⁶ = 20509597 is a prime.

Together with 20509663, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20509663) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10254830 + 10254831.

It is an arithmetic number, because the mean of its divisors is an integer number (10254831).

Almost surely, 2^20509661 is an apocalyptic number.

It is an amenable number.

20509661 is a deficient number, since it is larger than the sum of its proper divisors (1).

20509661 is an equidigital number, since it uses as much as digits as its factorization.

20509661 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 3240, while the sum is 29.

The square root of 20509661 is about 4528.7593223752. The cubic root of 20509661 is about 273.7281700377.

It can be divided in two parts, 2050 and 9661, that added together give a palindrome (11711).

The spelling of 20509661 in words is "twenty million, five hundred nine thousand, six hundred sixty-one".