20809991 has 2 divisors, whose sum is σ = 20809992. Its totient is φ = 20809990.

The previous prime is 20809961. The next prime is 20809993. The reversal of 20809991 is 19990802.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 20809991 - 2¹⁴ = 20793607 is a prime.

It is a Sophie Germain prime.

Together with 20809993, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20809993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10404995 + 10404996.

It is an arithmetic number, because the mean of its divisors is an integer number (10404996).

Almost surely, 2^20809991 is an apocalyptic number.

20809991 is a deficient number, since it is larger than the sum of its proper divisors (1).

20809991 is an equidigital number, since it uses as much as digits as its factorization.

20809991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 11664, while the sum is 38.

The square root of 20809991 is about 4561.7969047295. The cubic root of 20809991 is about 275.0577994211.

The spelling of 20809991 in words is "twenty million, eight hundred nine thousand, nine hundred ninety-one".