21501 has 6 divisors (see below), whose sum is σ = 31070. Its totient is φ = 14328.

The previous prime is 21499. The next prime is 21503. The reversal of 21501 is 10512.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21499) and next prime (21503).

It can be written as a sum of positive squares in only one way, i.e., 15876 + 5625 = 126^2 + 75^2 .

It is not a de Polignac number, because 21501 - 2¹ = 21499 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9), and also a Moran number because the ratio is a prime number: 2389 = 21501 / (2 + 1 + 5 + 0 + 1).

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21503) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1186 + ... + 1203.

2²¹⁵⁰¹ is an apocalyptic number.

It is an amenable number.

21501 is a deficient number, since it is larger than the sum of its proper divisors (9569).

21501 is a wasteful number, since it uses less digits than its factorization.

21501 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2395 (or 2392 counting only the distinct ones).

The product of its (nonzero) digits is 10, while the sum is 9.

The square root of 21501 is about 146.6321929182. The cubic root of 21501 is about 27.8069199343.

The spelling of 21501 in words is "twenty-one thousand, five hundred one".