261661721615 has 24 divisors (see below), whose sum is σ = 368874857952. Its totient is φ = 177648676800.

The previous prime is 261661721611. The next prime is 261661721693. The reversal of 261661721615 is 516127166162.

It is a happy number.

It is not a de Polignac number, because 261661721615 - 2² = 261661721611 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (261661721611) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 5262419 + ... + 5311908.

It is an arithmetic number, because the mean of its divisors is an integer number (15369785748).

Almost surely, 2^261661721615 is an apocalyptic number.

261661721615 is a deficient number, since it is larger than the sum of its proper divisors (107213136337).

261661721615 is a wasteful number, since it uses less digits than its factorization.

261661721615 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10574447 (or 10574440 counting only the distinct ones).

The product of its digits is 181440, while the sum is 44.

Adding to 261661721615 its reverse (516127166162), we get a palindrome (777788887777).

The spelling of 261661721615 in words is "two hundred sixty-one billion, six hundred sixty-one million, seven hundred twenty-one thousand, six hundred fifteen".