21709621 has 2 divisors, whose sum is σ = 21709622. Its totient is φ = 21709620.

The previous prime is 21709613. The next prime is 21709651. The reversal of 21709621 is 12690712.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15760900 + 5948721 = 3970^2 + 2439^2 .

It is a cyclic number.

It is not a de Polignac number, because 21709621 - 2³ = 21709613 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21709621.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21709601) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10854810 + 10854811.

It is an arithmetic number, because the mean of its divisors is an integer number (10854811).

Almost surely, 2^21709621 is an apocalyptic number.

It is an amenable number.

21709621 is a deficient number, since it is larger than the sum of its proper divisors (1).

21709621 is an equidigital number, since it uses as much as digits as its factorization.

21709621 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1512, while the sum is 28.

The square root of 21709621 is about 4659.3584322308. The cubic root of 21709621 is about 278.9656608260.

The spelling of 21709621 in words is "twenty-one million, seven hundred nine thousand, six hundred twenty-one".