61991 has 2 divisors, whose sum is σ = 61992.
Its totient is φ = 61990.
The previous prime is 61987. The next prime is 62003. The reversal of 61991 is 19916.
It is a weak prime.
It is a cyclic number.
It is not a de Polignac number, because 61991 - 22 = 61987 is a prime.
It is a Sophie Germain prime.
It is a Chen prime.
It is a plaindrome in base 15.
It is a self number, because there is not a number n which added to its sum of digits gives 61991.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (61961) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 30995 + 30996.
It is an arithmetic number, because the mean of its divisors is an integer number (30996).
261991 is an apocalyptic number.
61991 is a deficient number, since it is larger than the sum of its proper divisors (1).
61991 is an equidigital number, since it uses as much as digits as its factorization.
61991 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 486, while the sum is 26.
The square root of 61991 is about 248.9799188690.
The cubic root of 61991 is about 39.5770008953.
The spelling of 61991 in words is "sixty-one thousand, nine hundred ninety-one".