21709409 has 2 divisors, whose sum is σ = 21709410. Its totient is φ = 21709408.

The previous prime is 21709403. The next prime is 21709411. The reversal of 21709409 is 90490712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17181025 + 4528384 = 4145^2 + 2128^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21709409 is a prime.

Together with 21709411, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21709403) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10854704 + 10854705.

It is an arithmetic number, because the mean of its divisors is an integer number (10854705).

Almost surely, 2^21709409 is an apocalyptic number.

It is an amenable number.

21709409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21709409 is an equidigital number, since it uses as much as digits as its factorization.

21709409 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4536, while the sum is 32.

The square root of 21709409 is about 4659.3356822620. The cubic root of 21709409 is about 278.9647527660.

The spelling of 21709409 in words is "twenty-one million, seven hundred nine thousand, four hundred nine".