21905156561 has 2 divisors, whose sum is σ = 21905156562. Its totient is φ = 21905156560.

The previous prime is 21905156531. The next prime is 21905156587. The reversal of 21905156561 is 16565150912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21526464961 + 378691600 = 146719^2 + 19460^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21905156561 is a prime.

It is not a weakly prime, because it can be changed into another prime (21905156531) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10952578280 + 10952578281.

It is an arithmetic number, because the mean of its divisors is an integer number (10952578281).

Almost surely, 2^21905156561 is an apocalyptic number.

It is an amenable number.

21905156561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21905156561 is an equidigital number, since it uses as much as digits as its factorization.

21905156561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 81000, while the sum is 41.

The spelling of 21905156561 in words is "twenty-one billion, nine hundred five million, one hundred fifty-six thousand, five hundred sixty-one".