51991981 has 4 divisors (see below), whose sum is σ = 52007200. Its totient is φ = 51976764.

The previous prime is 51991969. The next prime is 51991997. The reversal of 51991981 is 18919915.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 51991981 - 2⁵ = 51991949 is a prime.

It is a super-2 number, since 2×51991981² = 5406332176608722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (51991081) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 160 + ... + 10198.

It is an arithmetic number, because the mean of its divisors is an integer number (13001800).

Almost surely, 2^51991981 is an apocalyptic number.

It is an amenable number.

51991981 is a deficient number, since it is larger than the sum of its proper divisors (15219).

51991981 is a wasteful number, since it uses less digits than its factorization.

51991981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15218.

The product of its digits is 29160, while the sum is 43.

The square root of 51991981 is about 7210.5465118811. The cubic root of 51991981 is about 373.2319281525.

The spelling of 51991981 in words is "fifty-one million, nine hundred ninety-one thousand, nine hundred eighty-one".