991521173 has 2 divisors, whose sum is σ = 991521174. Its totient is φ = 991521172.

The previous prime is 991521149. The next prime is 991521187. The reversal of 991521173 is 371125199.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 866595844 + 124925329 = 29438^2 + 11177^2 .

It is a cyclic number.

It is not a de Polignac number, because 991521173 - 2¹⁶ = 991455637 is a prime.

It is a super-2 number, since 2×991521173² = 1966228473014591858, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991523173) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495760586 + 495760587.

It is an arithmetic number, because the mean of its divisors is an integer number (495760587).

Almost surely, 2^991521173 is an apocalyptic number.

It is an amenable number.

991521173 is a deficient number, since it is larger than the sum of its proper divisors (1).

991521173 is an equidigital number, since it uses as much as digits as its factorization.

991521173 is an evil number, because the sum of its binary digits is even.

The product of its digits is 17010, while the sum is 38.

The square root of 991521173 is about 31488.4291923240. The cubic root of 991521173 is about 997.1656986588.

It can be divided in two parts, 9915 and 21173, that multiplied together give a triangular number (209930295 = T₂₀₄₉₀).

The spelling of 991521173 in words is "nine hundred ninety-one million, five hundred twenty-one thousand, one hundred seventy-three".