21591 has 6 divisors (see below), whose sum is σ = 31200. Its totient is φ = 14388.
The previous prime is 21589. The next prime is 21599. The reversal of 21591 is 19512.
It is not a de Polignac number, because 21591 - 21 = 21589 is a prime.
21591 is a lucky number.
It is a congruent number.
It is not an unprimeable number, because it can be changed into a prime (21599) by changing a digit.
It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1191 + ... + 1208.
It is an arithmetic number, because the mean of its divisors is an integer number (5200).
221591 is an apocalyptic number.
21591 is a deficient number, since it is larger than the sum of its proper divisors (9609).
21591 is a wasteful number, since it uses less digits than its factorization.
21591 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 2405 (or 2402 counting only the distinct ones).
The product of its digits is 90, while the sum is 18.
The square root of 21591 is about 146.9387627551. The cubic root of 21591 is about 27.8456644736.
Adding to 21591 its sum of digits (18), we get a square (21609 = 1472).
The spelling of 21591 in words is "twenty-one thousand, five hundred ninety-one".
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