21128561 has 2 divisors, whose sum is σ = 21128562. Its totient is φ = 21128560.

The previous prime is 21128543. The next prime is 21128567. The reversal of 21128561 is 16582112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20611600 + 516961 = 4540^2 + 719^2 .

It is a cyclic number.

It is not a de Polignac number, because 21128561 - 2¹⁰ = 21127537 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21128561.

It is not a weakly prime, because it can be changed into another prime (21128567) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10564280 + 10564281.

It is an arithmetic number, because the mean of its divisors is an integer number (10564281).

Almost surely, 2^21128561 is an apocalyptic number.

It is an amenable number.

21128561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21128561 is an equidigital number, since it uses as much as digits as its factorization.

21128561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 960, while the sum is 26.

The square root of 21128561 is about 4596.5814471192. The cubic root of 21128561 is about 276.4542727241.

The spelling of 21128561 in words is "twenty-one million, one hundred twenty-eight thousand, five hundred sixty-one".