661561281 has 12 divisors (see below), whose sum is σ = 978896100. Its totient is φ = 430283520.

The previous prime is 661561279. The next prime is 661561319. The reversal of 661561281 is 182165166.

It can be written as a sum of positive squares in 2 ways, for example, as 209786256 + 451775025 = 14484^2 + 21255^2 .

It is not a de Polignac number, because 661561281 - 2¹ = 661561279 is a prime.

It is a super-3 number, since 3×661561281³ (a number of 27 digits) contains 333 as substring. Note that it is a super-d number also for d = 2.

It is not an unprimeable number, because it can be changed into a prime (661561781) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 896056 + ... + 896793.

It is an arithmetic number, because the mean of its divisors is an integer number (81574675).

Almost surely, 2^661561281 is an apocalyptic number.

It is an amenable number.

661561281 is a deficient number, since it is larger than the sum of its proper divisors (317334819).

661561281 is a wasteful number, since it uses less digits than its factorization.

661561281 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1792896 (or 1792893 counting only the distinct ones).

The product of its digits is 17280, while the sum is 36.

The square root of 661561281 is about 25720.8335984664. The cubic root of 661561281 is about 871.3447653412.

It can be divided in two parts, 6615 and 61281, that added together give a triangular number (67896 = T₃₆₈).

The spelling of 661561281 in words is "six hundred sixty-one million, five hundred sixty-one thousand, two hundred eighty-one".