21872309 has 2 divisors, whose sum is σ = 21872310. Its totient is φ = 21872308.

The previous prime is 21872281. The next prime is 21872317. The reversal of 21872309 is 90327812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21022225 + 850084 = 4585^2 + 922^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21872309 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21872369) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10936154 + 10936155.

It is an arithmetic number, because the mean of its divisors is an integer number (10936155).

Almost surely, 2^21872309 is an apocalyptic number.

It is an amenable number.

21872309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21872309 is an equidigital number, since it uses as much as digits as its factorization.

21872309 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6048, while the sum is 32.

The square root of 21872309 is about 4676.7840446187. The cubic root of 21872309 is about 279.6607668903.

It can be divided in two parts, 218 and 72309, that added together give a palindrome (72527).

The spelling of 21872309 in words is "twenty-one million, eight hundred seventy-two thousand, three hundred nine".