661645 has 4 divisors (see below), whose sum is σ = 793980. Its totient is φ = 529312.

The previous prime is 661621. The next prime is 661663. The reversal of 661645 is 546166.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 546166 = 2 ⋅273083.

It can be written as a sum of positive squares in 2 ways, for example, as 218089 + 443556 = 467^2 + 666^2 .

It is a cyclic number.

It is not a de Polignac number, because 661645 - 2⁵ = 661613 is a prime.

It is a super-3 number, since 3×661645³ = 868953145142733375, which contains 333 as substring.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 66160 + ... + 66169.

It is an arithmetic number, because the mean of its divisors is an integer number (198495).

2⁶⁶¹⁶⁴⁵ is an apocalyptic number.

It is an amenable number.

661645 is a deficient number, since it is larger than the sum of its proper divisors (132335).

661645 is a wasteful number, since it uses less digits than its factorization.

661645 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 132334.

The product of its digits is 4320, while the sum is 28.

The square root of 661645 is about 813.4156379121. The cubic root of 661645 is about 87.1381519324.

It can be divided in two parts, 66 and 1645, that added together give a triangular number (1711 = T₅₈).

The spelling of 661645 in words is "six hundred sixty-one thousand, six hundred forty-five".