21009997 has 2 divisors, whose sum is σ = 21009998. Its totient is φ = 21009996.

The previous prime is 21009983. The next prime is 21010013. The reversal of 21009997 is 79990012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18610596 + 2399401 = 4314^2 + 1549^2 .

It is a cyclic number.

It is not a de Polignac number, because 21009997 - 2⁷ = 21009869 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21009907) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504998 + 10504999.

It is an arithmetic number, because the mean of its divisors is an integer number (10504999).

Almost surely, 2^21009997 is an apocalyptic number.

It is an amenable number.

21009997 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009997 is an equidigital number, since it uses as much as digits as its factorization.

21009997 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 10206, while the sum is 37.

The square root of 21009997 is about 4583.6663272974. Note that the first 3 decimals coincide. The cubic root of 21009997 is about 275.9361900024.

The spelling of 21009997 in words is "twenty-one million, nine thousand, nine hundred ninety-seven".