661781 has 4 divisors (see below), whose sum is σ = 677964. Its totient is φ = 645600.

The previous prime is 661777. The next prime is 661823. The reversal of 661781 is 187166.

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 128881 + 532900 = 359^2 + 730^2 .

It is a cyclic number.

It is not a de Polignac number, because 661781 - 2² = 661777 is a prime.

It is a super-2 number, since 2×661781² = 875908183922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 661781.

It is not an unprimeable number, because it can be changed into a prime (661721) 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, 8030 + ... + 8111.

It is an arithmetic number, because the mean of its divisors is an integer number (169491).

2⁶⁶¹⁷⁸¹ is an apocalyptic number.

It is an amenable number.

661781 is a deficient number, since it is larger than the sum of its proper divisors (16183).

661781 is a wasteful number, since it uses less digits than its factorization.

661781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16182.

The product of its digits is 2016, while the sum is 29.

The square root of 661781 is about 813.4992317144. The cubic root of 661781 is about 87.1441218895.

The spelling of 661781 in words is "six hundred sixty-one thousand, seven hundred eighty-one".