20661 has 8 divisors (see below), whose sum is σ = 28224. Its totient is φ = 13440.

The previous prime is 20641. The next prime is 20663. The reversal of 20661 is 16602.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 20661 - 2⁷ = 20533 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 20661.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20663) by changing a digit.

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, 165 + ... + 261.

It is an arithmetic number, because the mean of its divisors is an integer number (3528).

2²⁰⁶⁶¹ is an apocalyptic number.

It is an amenable number.

20661 is a deficient number, since it is larger than the sum of its proper divisors (7563).

20661 is an equidigital number, since it uses as much as digits as its factorization.

20661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 171.

The product of its (nonzero) digits is 72, while the sum is 15.

The square root of 20661 is about 143.7393474314. The cubic root of 20661 is about 27.4399792887.

The spelling of 20661 in words is "twenty thousand, six hundred sixty-one".