21209561 has 2 divisors, whose sum is σ = 21209562. Its totient is φ = 21209560.

The previous prime is 21209537. The next prime is 21209563. The reversal of 21209561 is 16590212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19193161 + 2016400 = 4381^2 + 1420^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21209561 is a prime.

It is a Sophie Germain prime.

Together with 21209563, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 21209561.

It is not a weakly prime, because it can be changed into another prime (21209563) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10604780 + 10604781.

It is an arithmetic number, because the mean of its divisors is an integer number (10604781).

Almost surely, 2^21209561 is an apocalyptic number.

It is an amenable number.

21209561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21209561 is an equidigital number, since it uses as much as digits as its factorization.

21209561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1080, while the sum is 26.

The square root of 21209561 is about 4605.3839145070. The cubic root of 21209561 is about 276.8071006857.

Adding to 21209561 its reverse (16590212), we get a palindrome (37799773).

The spelling of 21209561 in words is "twenty-one million, two hundred nine thousand, five hundred sixty-one".