21509413 has 2 divisors, whose sum is σ = 21509414. Its totient is φ = 21509412.

The previous prime is 21509359. The next prime is 21509419. The reversal of 21509413 is 31490512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16136289 + 5373124 = 4017^2 + 2318^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21509413 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21509419) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10754706 + 10754707.

It is an arithmetic number, because the mean of its divisors is an integer number (10754707).

Almost surely, 2^21509413 is an apocalyptic number.

It is an amenable number.

21509413 is a deficient number, since it is larger than the sum of its proper divisors (1).

21509413 is an equidigital number, since it uses as much as digits as its factorization.

21509413 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1080, while the sum is 25.

The square root of 21509413 is about 4637.8241665678. The cubic root of 21509413 is about 278.1054626347.

Adding to 21509413 its reverse (31490512), we get a palindrome (52999925).

The spelling of 21509413 in words is "twenty-one million, five hundred nine thousand, four hundred thirteen".