21684209 has 2 divisors, whose sum is σ = 21684210. Its totient is φ = 21684208.

The previous prime is 21684191. The next prime is 21684217. The reversal of 21684209 is 90248612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18515809 + 3168400 = 4303^2 + 1780^2 .

It is a cyclic number.

It is not a de Polignac number, because 21684209 - 2¹⁶ = 21618673 is a prime.

It is equal to p_1370498 and since 21684209 and 1370498 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21684409) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10842104 + 10842105.

It is an arithmetic number, because the mean of its divisors is an integer number (10842105).

Almost surely, 2^21684209 is an apocalyptic number.

It is an amenable number.

21684209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21684209 is an equidigital number, since it uses as much as digits as its factorization.

21684209 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6912, while the sum is 32.

The square root of 21684209 is about 4656.6306488705. The cubic root of 21684209 is about 278.8567714202.

The spelling of 21684209 in words is "twenty-one million, six hundred eighty-four thousand, two hundred nine".