21537 has 6 divisors (see below), whose sum is σ = 31122. Its totient is φ = 14352.

The previous prime is 21529. The next prime is 21557. The reversal of 21537 is 73512.

It can be written as a sum of positive squares in only one way, i.e., 12321 + 9216 = 111^2 + 96^2 .

It is not a de Polignac number, because 21537 - 2³ = 21529 is a prime.

21537 is a lucky number.

It is a nialpdrome in base 16.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21537.

It is not an unprimeable number, because it can be changed into a prime (21517) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1188 + ... + 1205.

It is an arithmetic number, because the mean of its divisors is an integer number (5187).

2²¹⁵³⁷ is an apocalyptic number.

It is an amenable number.

21537 is a deficient number, since it is larger than the sum of its proper divisors (9585).

21537 is a wasteful number, since it uses less digits than its factorization.

21537 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2399 (or 2396 counting only the distinct ones).

The product of its digits is 210, while the sum is 18.

The square root of 21537 is about 146.7548977036. The cubic root of 21537 is about 27.8224307003.

It can be divided in two parts, 215 and 37, that added together give a palindrome (252).

The spelling of 21537 in words is "twenty-one thousand, five hundred thirty-seven".