21326561 has 2 divisors, whose sum is σ = 21326562. Its totient is φ = 21326560.

The previous prime is 21326537. The next prime is 21326563. The reversal of 21326561 is 16562312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11182336 + 10144225 = 3344^2 + 3185^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21326561 is a prime.

Together with 21326563, it forms a pair of twin primes.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21326561.

It is not a weakly prime, because it can be changed into another prime (21326563) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10663280 + 10663281.

It is an arithmetic number, because the mean of its divisors is an integer number (10663281).

Almost surely, 2^21326561 is an apocalyptic number.

It is an amenable number.

21326561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21326561 is an equidigital number, since it uses as much as digits as its factorization.

21326561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2160, while the sum is 26.

The square root of 21326561 is about 4618.0689687357. The cubic root of 21326561 is about 277.3151586737.

Adding to 21326561 its reverse (16562312), we get a palindrome (37888873).

The spelling of 21326561 in words is "twenty-one million, three hundred twenty-six thousand, five hundred sixty-one".