21155240261 has 2 divisors, whose sum is σ = 21155240262. Its totient is φ = 21155240260.

The previous prime is 21155240177. The next prime is 21155240263. The reversal of 21155240261 is 16204255112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18218250625 + 2936989636 = 134975^2 + 54194^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21155240261 is a prime.

Together with 21155240263, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21155240263) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10577620130 + 10577620131.

It is an arithmetic number, because the mean of its divisors is an integer number (10577620131).

Almost surely, 2^21155240261 is an apocalyptic number.

It is an amenable number.

21155240261 is a deficient number, since it is larger than the sum of its proper divisors (1).

21155240261 is an equidigital number, since it uses as much as digits as its factorization.

21155240261 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4800, while the sum is 29.

Adding to 21155240261 its reverse (16204255112), we get a palindrome (37359495373).

The spelling of 21155240261 in words is "twenty-one billion, one hundred fifty-five million, two hundred forty thousand, two hundred sixty-one".