991638155 has 4 divisors (see below), whose sum is σ = 1189965792. Its totient is φ = 793310520.

The previous prime is 991638133. The next prime is 991638157. The reversal of 991638155 is 551836199.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 551836199 = 103 ⋅5357633.

It is not a de Polignac number, because 991638155 - 2⁶ = 991638091 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (991638157) 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, 99163811 + ... + 99163820.

It is an arithmetic number, because the mean of its divisors is an integer number (297491448).

Almost surely, 2^991638155 is an apocalyptic number.

991638155 is a deficient number, since it is larger than the sum of its proper divisors (198327637).

991638155 is a wasteful number, since it uses less digits than its factorization.

991638155 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 198327636.

The product of its digits is 291600, while the sum is 47.

The square root of 991638155 is about 31490.2866770057. The cubic root of 991638155 is about 997.2049131014.

The spelling of 991638155 in words is "nine hundred ninety-one million, six hundred thirty-eight thousand, one hundred fifty-five".