991947978 has 64 divisors (see below), whose sum is σ = 2539192320. Its totient is φ = 281168496.

The previous prime is 991947961. The next prime is 991947997. The reversal of 991947978 is 879749199.

991947978 is a `hidden beast` number, since 9 + 91 + 9 + 479 + 78 = 666.

It is a Harshad number since it is a multiple of its sum of digits (63).

It is a Curzon number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 37675 + ... + 58337.

It is an arithmetic number, because the mean of its divisors is an integer number (39674880).

Almost surely, 2^991947978 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 991947978, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1269596160).

991947978 is an abundant number, since it is smaller than the sum of its proper divisors (1547244342).

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

991947978 is a wasteful number, since it uses less digits than its factorization.

991947978 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20808 (or 20802 counting only the distinct ones).

The product of its digits is 10287648, while the sum is 63.

The square root of 991947978 is about 31495.2056351439. The cubic root of 991947978 is about 997.3087563718.

The spelling of 991947978 in words is "nine hundred ninety-one million, nine hundred forty-seven thousand, nine hundred seventy-eight".