991531 has 2 divisors, whose sum is σ = 991532. Its totient is φ = 991530.

The previous prime is 991511. The next prime is 991541. The reversal of 991531 is 135199.

991531 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

991531 is an esthetic number in base 3, because in such base its adjacent digits differ by 1.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 991531 - 2⁵ = 991499 is a prime.

It is a super-2 number, since 2×991531² = 1966267447922, which contains 22 as substring.

991531 is a lucky number.

It is a self number, because there is not a number n which added to its sum of digits gives 991531.

It is not a weakly prime, because it can be changed into another prime (991511) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495765 + 495766.

It is an arithmetic number, because the mean of its divisors is an integer number (495766).

2⁹⁹¹⁵³¹ is an apocalyptic number.

991531 is a deficient number, since it is larger than the sum of its proper divisors (1).

991531 is an equidigital number, since it uses as much as digits as its factorization.

991531 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1215, while the sum is 28.

The square root of 991531 is about 995.7564963383. The cubic root of 991531 is about 99.7168992962.

The spelling of 991531 in words is "nine hundred ninety-one thousand, five hundred thirty-one".