21980 has 24 divisors (see below), whose sum is σ = 53088. Its totient is φ = 7488.

The previous prime is 21977. The next prime is 21991. The reversal of 21980 is 8912.

21980 = T₉ + T₁₀ + ... + T₅₀.

21980 = 53² + 54² + ... + 59².

21980 is nontrivially palindromic in base 13.

It is a Harshad number since it is a multiple of its sum of digits (20).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 62 + ... + 218.

It is an arithmetic number, because the mean of its divisors is an integer number (2212).

2²¹⁹⁸⁰ is an apocalyptic number.

21980 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 21980, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26544).

21980 is an abundant number, since it is smaller than the sum of its proper divisors (31108).

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

21980 is a wasteful number, since it uses less digits than its factorization.

21980 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 173 (or 171 counting only the distinct ones).

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 21980 is about 148.2565344260. The cubic root of 21980 is about 28.0118997039.

It can be divided in two parts, 21 and 980, that added together give a palindrome (1001).

The spelling of 21980 in words is "twenty-one thousand, nine hundred eighty".