651991 has 4 divisors (see below), whose sum is σ = 654192. Its totient is φ = 649792.

The previous prime is 651971. The next prime is 651997. The reversal of 651991 is 199156.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 651991 - 2⁷ = 651863 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (651997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 571 + ... + 1276.

It is an arithmetic number, because the mean of its divisors is an integer number (163548).

2⁶⁵¹⁹⁹¹ is an apocalyptic number.

651991 is a deficient number, since it is larger than the sum of its proper divisors (2201).

651991 is a wasteful number, since it uses less digits than its factorization.

651991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2200.

The product of its digits is 2430, while the sum is 31.

The square root of 651991 is about 807.4595965124. The cubic root of 651991 is about 86.7122656164.

The spelling of 651991 in words is "six hundred fifty-one thousand, nine hundred ninety-one".