261891821 has 2 divisors, whose sum is σ = 261891822. Its totient is φ = 261891820.

The previous prime is 261891797. The next prime is 261891827. The reversal of 261891821 is 128198162.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 145467721 + 116424100 = 12061^2 + 10790^2 .

It is a cyclic number.

It is not a de Polignac number, because 261891821 - 2¹⁴ = 261875437 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (261891827) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 130945910 + 130945911.

It is an arithmetic number, because the mean of its divisors is an integer number (130945911).

Almost surely, 2^261891821 is an apocalyptic number.

It is an amenable number.

261891821 is a deficient number, since it is larger than the sum of its proper divisors (1).

261891821 is an equidigital number, since it uses as much as digits as its factorization.

261891821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 13824, while the sum is 38.

The square root of 261891821 is about 16183.0720507572. The cubic root of 261891821 is about 639.7947103617.

The spelling of 261891821 in words is "two hundred sixty-one million, eight hundred ninety-one thousand, eight hundred twenty-one".