20161991 has 2 divisors, whose sum is σ = 20161992. Its totient is φ = 20161990.

The previous prime is 20161987. The next prime is 20161993. The reversal of 20161991 is 19916102.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 20161991 - 2² = 20161987 is a prime.

Together with 20161993, 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 (20161993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10080995 + 10080996.

It is an arithmetic number, because the mean of its divisors is an integer number (10080996).

Almost surely, 2^20161991 is an apocalyptic number.

20161991 is a deficient number, since it is larger than the sum of its proper divisors (1).

20161991 is an equidigital number, since it uses as much as digits as its factorization.

20161991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 972, while the sum is 29.

The square root of 20161991 is about 4490.2105741268. The cubic root of 20161991 is about 272.1726439645.

The spelling of 20161991 in words is "twenty million, one hundred sixty-one thousand, nine hundred ninety-one".