20851991 has 2 divisors, whose sum is σ = 20851992. Its totient is φ = 20851990.

The previous prime is 20851969. The next prime is 20852023. The reversal of 20851991 is 19915802.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 20851991 - 2⁶ = 20851927 is a prime.

It is a Sophie Germain prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20851951) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10425995 + 10425996.

It is an arithmetic number, because the mean of its divisors is an integer number (10425996).

Almost surely, 2^20851991 is an apocalyptic number.

20851991 is a deficient number, since it is larger than the sum of its proper divisors (1).

20851991 is an equidigital number, since it uses as much as digits as its factorization.

20851991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6480, while the sum is 35.

The square root of 20851991 is about 4566.3980334614. The cubic root of 20851991 is about 275.2427212429.

The spelling of 20851991 in words is "twenty million, eight hundred fifty-one thousand, nine hundred ninety-one".