21716809 has 2 divisors, whose sum is σ = 21716810. Its totient is φ = 21716808.

The previous prime is 21716767. The next prime is 21716831. The reversal of 21716809 is 90861712.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16426809 + 5290000 = 4053^2 + 2300^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21716809 is a prime.

It is not a weakly prime, because it can be changed into another prime (21716869) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10858404 + 10858405.

It is an arithmetic number, because the mean of its divisors is an integer number (10858405).

Almost surely, 2^21716809 is an apocalyptic number.

It is an amenable number.

21716809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21716809 is an equidigital number, since it uses as much as digits as its factorization.

21716809 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 34.

The square root of 21716809 is about 4660.1297192246. The cubic root of 21716809 is about 278.9964457010.

The spelling of 21716809 in words is "twenty-one million, seven hundred sixteen thousand, eight hundred nine".