261891631 has 2 divisors, whose sum is σ = 261891632. Its totient is φ = 261891630.

The previous prime is 261891607. The next prime is 261891673. The reversal of 261891631 is 136198162.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 261891631 - 2⁵ = 261891599 is a prime.

It is a super-2 number, since 2×261891631² = 137174452775680322, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (261891691) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 130945815 + 130945816.

It is an arithmetic number, because the mean of its divisors is an integer number (130945816).

Almost surely, 2^261891631 is an apocalyptic number.

261891631 is a deficient number, since it is larger than the sum of its proper divisors (1).

261891631 is an equidigital number, since it uses as much as digits as its factorization.

261891631 is an evil number, because the sum of its binary digits is even.

The product of its digits is 15552, while the sum is 37.

The square root of 261891631 is about 16183.0661804245. The cubic root of 261891631 is about 639.7945556400.

It can be divided in two parts, 2618 and 91631, that added together give a palindrome (94249).

The spelling of 261891631 in words is "two hundred sixty-one million, eight hundred ninety-one thousand, six hundred thirty-one".