21506 has 4 divisors (see below), whose sum is σ = 32262. Its totient is φ = 10752.

The previous prime is 21503. The next prime is 21517. The reversal of 21506 is 60512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 13225 + 8281 = 115^2 + 91^2 .

It is a Curzon number.

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

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5375 + ... + 5378.

2²¹⁵⁰⁶ is an apocalyptic number.

21506 is a deficient number, since it is larger than the sum of its proper divisors (10756).

21506 is a wasteful number, since it uses less digits than its factorization.

21506 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10755.

The product of its (nonzero) digits is 60, while the sum is 14.

The square root of 21506 is about 146.6492413891. The cubic root of 21506 is about 27.8090752421.

The spelling of 21506 in words is "twenty-one thousand, five hundred six".