991578781 has 4 divisors (see below), whose sum is σ = 993014464. Its totient is φ = 990143100.

The previous prime is 991578779. The next prime is 991578793. The reversal of 991578781 is 187875199.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 991578781 - 2¹ = 991578779 is a prime.

It is a super-2 number, since 2×991578781² = 1966456957858891922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991578751) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 716805 + ... + 718186.

It is an arithmetic number, because the mean of its divisors is an integer number (248253616).

Almost surely, 2^991578781 is an apocalyptic number.

It is an amenable number.

991578781 is a deficient number, since it is larger than the sum of its proper divisors (1435683).

991578781 is a wasteful number, since it uses less digits than its factorization.

991578781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1435682.

The product of its digits is 1270080, while the sum is 55.

The square root of 991578781 is about 31489.3439277480. The cubic root of 991578781 is about 997.1850102683.

The spelling of 991578781 in words is "nine hundred ninety-one million, five hundred seventy-eight thousand, seven hundred eighty-one".