21690 has 24 divisors (see below), whose sum is σ = 56628.
Its totient is φ = 5760.
The previous prime is 21683. The next prime is 21701. The reversal of 21690 is 9612.
It can be written as a sum of positive squares in 2 ways, for example, as 21609 + 81 = 147^2 + 9^2
It is a super-2 number, since 2×216902 = 940912200, which contains 22 as substring.
It is a Harshad number since it is a multiple of its sum of digits (18).
It is an alternating number because its digits alternate between even and odd.
It is a nialpdrome in base 15.
It is an unprimeable number.
It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 31 + ... + 210.
221690 is an apocalyptic number.
21690 is an abundant number, since it is smaller than the sum of its proper divisors (34938).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28314).
21690 is a wasteful number, since it uses less digits than its factorization.
21690 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 254 (or 251 counting only the distinct ones).
The product of its (nonzero) digits is 108, while the sum is 18.
The square root of 21690 is about 147.2752525036.
The cubic root of 21690 is about 27.8881593105.
Note that the first 3 decimals are identical.
The spelling of 21690 in words is "twenty-one thousand, six hundred ninety".