21566821 has 2 divisors, whose sum is σ = 21566822. Its totient is φ = 21566820.

The previous prime is 21566777. The next prime is 21566827. The reversal of 21566821 is 12866512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21252100 + 314721 = 4610^2 + 561^2 .

It is a cyclic number.

It is not a de Polignac number, because 21566821 - 2¹⁹ = 21042533 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21566827) 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, 10783410 + 10783411.

It is an arithmetic number, because the mean of its divisors is an integer number (10783411).

Almost surely, 2^21566821 is an apocalyptic number.

It is an amenable number.

21566821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21566821 is an equidigital number, since it uses as much as digits as its factorization.

21566821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5760, while the sum is 31.

The square root of 21566821 is about 4644.0091515844. The cubic root of 21566821 is about 278.3526613165.

The spelling of 21566821 in words is "twenty-one million, five hundred sixty-six thousand, eight hundred twenty-one".