21780 has 54 divisors (see below), whose sum is σ = 72618. Its totient is φ = 5280.

The previous prime is 21773. The next prime is 21787. The reversal of 21780 is 8712.

It is an interprime number because it is at equal distance from previous prime (21773) and next prime (21787).

It can be written as a sum of positive squares in only one way, i.e., 17424 + 4356 = 132^2 + 66^2 .

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21787) by changing a digit.

21780 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 1975 + ... + 1985.

2²¹⁷⁸⁰ is an apocalyptic number.

21780 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 21780, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36309).

21780 is an abundant number, since it is smaller than the sum of its proper divisors (50838).

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

21780 is a wasteful number, since it uses less digits than its factorization.

21780 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 37 (or 21 counting only the distinct ones).

The product of its (nonzero) digits is 112, while the sum is 18.

The square root of 21780 is about 147.5804865150. The cubic root of 21780 is about 27.9266789166.

The spelling of 21780 in words is "twenty-one thousand, seven hundred eighty".