21002 has 4 divisors (see below), whose sum is σ = 31506. Its totient is φ = 10500.

The previous prime is 21001. The next prime is 21011. The reversal of 21002 is 20012.

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., 19321 + 1681 = 139^2 + 41^2 .

It is not an unprimeable number, because it can be changed into a prime (21001) 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 as a sum of consecutive naturals, namely, 5249 + ... + 5252.

2²¹⁰⁰² is an apocalyptic number.

21002 is a deficient number, since it is larger than the sum of its proper divisors (10504).

21002 is a wasteful number, since it uses less digits than its factorization.

21002 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10503.

The product of its (nonzero) digits is 4, while the sum is 5.

The square root of 21002 is about 144.9206679532. The cubic root of 21002 is about 27.5901175850.

Adding to 21002 its reverse (20012), we get a palindrome (41014).

Subtracting from 21002 its reverse (20012), we obtain a triangular number (990 = T₄₄).

Multiplying 21002 by its reverse (20012), we get a palindrome (420292024).

The spelling of 21002 in words is "twenty-one thousand, two", and thus it is an iban number.