51991 has 2 divisors, whose sum is σ = 51992. Its totient is φ = 51990.

The previous prime is 51977. The next prime is 52009. The reversal of 51991 is 19915.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 51991 - 2⁹ = 51479 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (51941) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 25995 + 25996.

It is an arithmetic number, because the mean of its divisors is an integer number (25996).

2⁵¹⁹⁹¹ is an apocalyptic number.

51991 is a deficient number, since it is larger than the sum of its proper divisors (1).

51991 is an equidigital number, since it uses as much as digits as its factorization.

51991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 405, while the sum is 25.

The square root of 51991 is about 228.0153503605. The cubic root of 51991 is about 37.3229580721.

The spelling of 51991 in words is "fifty-one thousand, nine hundred ninety-one".