1991081638 has 16 divisors (see below), whose sum is σ = 3675843360. Its totient is φ = 787680576.

The previous prime is 1991081627. The next prime is 1991081657. The reversal of 1991081638 is 8361801991.

It is a happy number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 5469823 + ... + 5470186.

It is an arithmetic number, because the mean of its divisors is an integer number (229740210).

Almost surely, 2^1991081638 is an apocalyptic number.

1991081638 is a deficient number, since it is larger than the sum of its proper divisors (1684761722).

1991081638 is a wasteful number, since it uses less digits than its factorization.

1991081638 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10940031.

The product of its (nonzero) digits is 93312, while the sum is 46.

The square root of 1991081638 is about 44621.5378264802. The cubic root of 1991081638 is about 1258.0455206824.

The spelling of 1991081638 in words is "one billion, nine hundred ninety-one million, eighty-one thousand, six hundred thirty-eight".