261666505155557 has 4 divisors (see below), whose sum is σ = 261674483555436. Its totient is φ = 261658526755680.

The previous prime is 261666505155553. The next prime is 261666505155569. The reversal of 261666505155557 is 755551505666162.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 189150482733601 + 72516022421956 = 13753199^2 + 8515634^2 .

It is a cyclic number.

It is not a de Polignac number, because 261666505155557 - 2² = 261666505155553 is a prime.

It is a super-3 number, since 3×261666505155557³ (a number of 44 digits) contains 333 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 261666505155493 and 261666505155502.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (261666505155553) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3989150744 + ... + 3989216337.

It is an arithmetic number, because the mean of its divisors is an integer number (65418620888859).

Almost surely, 2^{261666505155557} is an apocalyptic number.

It is an amenable number.

261666505155557 is a deficient number, since it is larger than the sum of its proper divisors (7978399879).

261666505155557 is an equidigital number, since it uses as much as digits as its factorization.

261666505155557 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7978399878.

The product of its (nonzero) digits is 283500000, while the sum is 65.

The spelling of 261666505155557 in words is "two hundred sixty-one trillion, six hundred sixty-six billion, five hundred five million, one hundred fifty-five thousand, five hundred fifty-seven".