21006 has 16 divisors (see below), whose sum is σ = 46800. Its totient is φ = 6984.

The previous prime is 21001. The next prime is 21011. The reversal of 21006 is 60012.

It is an interprime number because it is at equal distance from previous prime (21001) and next prime (21011).

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 141 + ... + 248.

It is an arithmetic number, because the mean of its divisors is an integer number (2925).

2²¹⁰⁰⁶ is an apocalyptic number.

21006 is an abundant number, since it is smaller than the sum of its proper divisors (25794).

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 (23400).

21006 is a wasteful number, since it uses less digits than its factorization.

21006 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 400 (or 394 counting only the distinct ones).

The product of its (nonzero) digits is 12, while the sum is 9.

The square root of 21006 is about 144.9344679502. The cubic root of 21006 is about 27.5918690604.

Adding to 21006 its reverse (60012), we get a palindrome (81018).

The spelling of 21006 in words is "twenty-one thousand, six".