21405997 has 2 divisors, whose sum is σ = 21405998. Its totient is φ = 21405996.

The previous prime is 21405961. The next prime is 21406001. The reversal of 21405997 is 79950412.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15888196 + 5517801 = 3986^2 + 2349^2 .

It is a cyclic number.

It is not a de Polignac number, because 21405997 - 2¹⁹ = 20881709 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21405497) 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, 10702998 + 10702999.

It is an arithmetic number, because the mean of its divisors is an integer number (10702999).

Almost surely, 2^21405997 is an apocalyptic number.

It is an amenable number.

21405997 is a deficient number, since it is larger than the sum of its proper divisors (1).

21405997 is an equidigital number, since it uses as much as digits as its factorization.

21405997 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 22680, while the sum is 37.

The square root of 21405997 is about 4626.6615393824. The cubic root of 21405997 is about 277.6590414785.

The spelling of 21405997 in words is "twenty-one million, four hundred five thousand, nine hundred ninety-seven".