991971001980 has 480 divisors, whose sum is σ = 3797751767040. Its totient is φ = 211853698560.

The previous prime is 991971001961. The next prime is 991971001993. The reversal of 991971001980 is 89100179199.

It is a happy number.

It is a hoax number, since the sum of its digits (54) coincides with the sum of the digits of its distinct prime factors.

It is a Harshad number since it is a multiple of its sum of digits (54).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 159 ways as a sum of consecutive naturals, for example, 10146009 + ... + 10243311.

It is an arithmetic number, because the mean of its divisors is an integer number (7911982848).

Almost surely, 2^991971001980 is an apocalyptic number.

991971001980 is a gapful number since it is divisible by the number (90) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 991971001980, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1898875883520).

991971001980 is an abundant number, since it is smaller than the sum of its proper divisors (2805780765060).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

991971001980 is a wasteful number, since it uses less digits than its factorization.

991971001980 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 97391 (or 97380 counting only the distinct ones).

The product of its (nonzero) digits is 367416, while the sum is 54.

The spelling of 991971001980 in words is "nine hundred ninety-one billion, nine hundred seventy-one million, one thousand, nine hundred eighty".