21615409 has 2 divisors, whose sum is σ = 21615410. Its totient is φ = 21615408.

The previous prime is 21615379. The next prime is 21615413. The reversal of 21615409 is 90451612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20223009 + 1392400 = 4497^2 + 1180^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21615409 is a prime.

It is not a weakly prime, because it can be changed into another prime (21615479) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10807704 + 10807705.

It is an arithmetic number, because the mean of its divisors is an integer number (10807705).

Almost surely, 2^21615409 is an apocalyptic number.

It is an amenable number.

21615409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21615409 is an equidigital number, since it uses as much as digits as its factorization.

21615409 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2160, while the sum is 28.

The square root of 21615409 is about 4649.2374643591. The cubic root of 21615409 is about 278.5615385738.

It can be divided in two parts, 216 and 15409, that added together give a 6-th power (15625 = 5⁶).

The spelling of 21615409 in words is "twenty-one million, six hundred fifteen thousand, four hundred nine".