20640 has 48 divisors (see below), whose sum is σ = 66528. Its totient is φ = 5376.

The previous prime is 20639. The next prime is 20641. The reversal of 20640 is 4602.

It is an interprime number because it is at equal distance from previous prime (20639) and next prime (20641).

It is a tau number, because it is divible by the number of its divisors (48).

It is a Harshad number since it is a multiple of its sum of digits (12).

It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.

It is a nialpdrome in base 12 and base 14.

It is a zygodrome in base 4 and base 14.

It is not an unprimeable number, because it can be changed into a prime (20641) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 459 + ... + 501.

It is an arithmetic number, because the mean of its divisors is an integer number (1386).

2²⁰⁶⁴⁰ is an apocalyptic number.

20640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 20640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33264).

20640 is an abundant number, since it is smaller than the sum of its proper divisors (45888).

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

20640 is a wasteful number, since it uses less digits than its factorization.

20640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 61 (or 53 counting only the distinct ones).

The product of its (nonzero) digits is 48, while the sum is 12.

The square root of 20640 is about 143.6662799685. Note that the first 3 decimals coincide. The cubic root of 20640 is about 27.4306794015.

The spelling of 20640 in words is "twenty thousand, six hundred forty".