661808 has 40 divisors (see below), whose sum is σ = 1547520. Its totient is φ = 267840.

The previous prime is 661777. The next prime is 661823. The reversal of 661808 is 808166.

It is an Ulam number.

It is an unprimeable number.

661808 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1973 + ... + 2283.

It is an arithmetic number, because the mean of its divisors is an integer number (38688).

2⁶⁶¹⁸⁰⁸ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 661808, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (773760).

661808 is an abundant number, since it is smaller than the sum of its proper divisors (885712).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

661808 is a wasteful number, since it uses less digits than its factorization.

661808 is an odious number, because the sum of its binary digits is odd.

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

The product of its (nonzero) digits is 2304, while the sum is 29.

The square root of 661808 is about 813.5158265209. The cubic root of 661808 is about 87.1453070042.

The spelling of 661808 in words is "six hundred sixty-one thousand, eight hundred eight".