21055561 has 2 divisors, whose sum is σ = 21055562. Its totient is φ = 21055560.

The previous prime is 21055543. The next prime is 21055579. The reversal of 21055561 is 16555012.

It is a balanced prime because it is at equal distance from previous prime (21055543) and next prime (21055579).

It can be written as a sum of positive squares in only one way, i.e., 15649936 + 5405625 = 3956^2 + 2325^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21055561 is a prime.

It is not a weakly prime, because it can be changed into another prime (21055501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10527780 + 10527781.

It is an arithmetic number, because the mean of its divisors is an integer number (10527781).

Almost surely, 2^21055561 is an apocalyptic number.

It is an amenable number.

21055561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21055561 is an equidigital number, since it uses as much as digits as its factorization.

21055561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1500, while the sum is 25.

The square root of 21055561 is about 4588.6338925654. The cubic root of 21055561 is about 276.1355185844.

The spelling of 21055561 in words is "twenty-one million, fifty-five thousand, five hundred sixty-one".