21635561 has 2 divisors, whose sum is σ = 21635562. Its totient is φ = 21635560.

The previous prime is 21635503. The next prime is 21635563. The reversal of 21635561 is 16553612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21576025 + 59536 = 4645^2 + 244^2 .

It is a cyclic number.

It is not a de Polignac number, because 21635561 - 2¹⁰ = 21634537 is a prime.

Together with 21635563, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21635563) 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, 10817780 + 10817781.

It is an arithmetic number, because the mean of its divisors is an integer number (10817781).

Almost surely, 2^21635561 is an apocalyptic number.

It is an amenable number.

21635561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21635561 is an equidigital number, since it uses as much as digits as its factorization.

21635561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5400, while the sum is 29.

The square root of 21635561 is about 4651.4041965841. The cubic root of 21635561 is about 278.6480791295.

The spelling of 21635561 in words is "twenty-one million, six hundred thirty-five thousand, five hundred sixty-one".