661581 has 16 divisors (see below), whose sum is σ = 993600. Its totient is φ = 435024.

The previous prime is 661553. The next prime is 661603. The reversal of 661581 is 185166.

It is not a de Polignac number, because 661581 - 2⁶ = 661517 is a prime.

It is a super-2 number, since 2×661581² = 875378839122, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661541) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2775 + ... + 3003.

It is an arithmetic number, because the mean of its divisors is an integer number (62100).

2⁶⁶¹⁵⁸¹ is an apocalyptic number.

It is an amenable number.

661581 is a deficient number, since it is larger than the sum of its proper divisors (332019).

661581 is a wasteful number, since it uses less digits than its factorization.

661581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 345 (or 339 counting only the distinct ones).

The product of its digits is 1440, while the sum is 27.

The square root of 661581 is about 813.3762966795. The cubic root of 661581 is about 87.1353422577.

The spelling of 661581 in words is "six hundred sixty-one thousand, five hundred eighty-one".