21003 has 4 divisors (see below), whose sum is σ = 28008. Its totient is φ = 14000.

The previous prime is 21001. The next prime is 21011. The reversal of 21003 is 30012.

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

It is a cyclic number.

It is not a de Polignac number, because 21003 - 2¹ = 21001 is a prime.

It is a super-2 number, since 2×21003² = 882252018, which contains 22 as substring.

It is an Ulam number.

It is a D-number.

It is a junction number, because it is equal to n+sod(n) for n = 20982 and 21000.

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 3 ways as a sum of consecutive naturals, for example, 3498 + ... + 3503.

It is an arithmetic number, because the mean of its divisors is an integer number (7002).

2²¹⁰⁰³ is an apocalyptic number.

21003 is a deficient number, since it is larger than the sum of its proper divisors (7005).

21003 is an equidigital number, since it uses as much as digits as its factorization.

21003 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7004.

The product of its (nonzero) digits is 6, while the sum is 6.

The square root of 21003 is about 144.9241180756. The cubic root of 21003 is about 27.5905554747.

Adding to 21003 its reverse (30012), we get a palindrome (51015).

Subtracting 21003 from its reverse (30012), we obtain a palindrome (9009).

The spelling of 21003 in words is "twenty-one thousand, three", and thus it is an iban number.