21717989 has 2 divisors, whose sum is σ = 21717990. Its totient is φ = 21717988.

The previous prime is 21717979. The next prime is 21718033. The reversal of 21717989 is 98971712.

It is an a-pointer prime, because the next prime (21718033) can be obtained adding 21717989 to its sum of digits (44).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21696964 + 21025 = 4658^2 + 145^2 .

It is a cyclic number.

It is not a de Polignac number, because 21717989 - 2⁴ = 21717973 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21717919) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10858994 + 10858995.

It is an arithmetic number, because the mean of its divisors is an integer number (10858995).

Almost surely, 2^21717989 is an apocalyptic number.

It is an amenable number.

21717989 is a deficient number, since it is larger than the sum of its proper divisors (1).

21717989 is an equidigital number, since it uses as much as digits as its factorization.

21717989 is an evil number, because the sum of its binary digits is even.

The product of its digits is 63504, while the sum is 44.

The square root of 21717989 is about 4660.2563234226. The cubic root of 21717989 is about 279.0014987737.

The spelling of 21717989 in words is "twenty-one million, seven hundred seventeen thousand, nine hundred eighty-nine".