21287209 has 2 divisors, whose sum is σ = 21287210. Its totient is φ = 21287208.

The previous prime is 21287143. The next prime is 21287219. The reversal of 21287209 is 90278212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19669225 + 1617984 = 4435^2 + 1272^2 .

It is a cyclic number.

It is not a de Polignac number, because 21287209 - 2⁷ = 21287081 is a prime.

It is not a weakly prime, because it can be changed into another prime (21287219) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10643604 + 10643605.

It is an arithmetic number, because the mean of its divisors is an integer number (10643605).

Almost surely, 2^21287209 is an apocalyptic number.

It is an amenable number.

21287209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21287209 is an equidigital number, since it uses as much as digits as its factorization.

21287209 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4032, while the sum is 31.

The square root of 21287209 is about 4613.8063461745. The cubic root of 21287209 is about 277.1444853489.

The spelling of 21287209 in words is "twenty-one million, two hundred eighty-seven thousand, two hundred nine".