63999991 has 16 divisors (see below), whose sum is σ = 71705088. Its totient is φ = 56628000.

The previous prime is 63999979. The next prime is 64000031. The reversal of 63999991 is 19999936.

It is not a de Polignac number, because 63999991 - 2¹⁹ = 63475703 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (63999931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 87670 + ... + 88396.

It is an arithmetic number, because the mean of its divisors is an integer number (4481568).

Almost surely, 2^63999991 is an apocalyptic number.

63999991 is a deficient number, since it is larger than the sum of its proper divisors (7705097).

63999991 is a wasteful number, since it uses less digits than its factorization.

63999991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 942.

The product of its digits is 1062882, while the sum is 55.

The square root of 63999991 is about 7999.9994375000. Note that the first 3 decimals coincide. The cubic root of 63999991 is about 399.9999812500. Note that the first 4 decimals are identical.

Adding to 63999991 its sum of digits (55), we get a palindrome (64000046).

Subtracting from 63999991 its sum of digits (55), we obtain a palindrome (63999936).

The spelling of 63999991 in words is "sixty-three million, nine hundred ninety-nine thousand, nine hundred ninety-one".