21015 has 12 divisors (see below), whose sum is σ = 36504. Its totient is φ = 11184.

The previous prime is 21013. The next prime is 21017. The reversal of 21015 is 51012.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21013) and next prime (21017).

It is not a de Polignac number, because 21015 - 2¹ = 21013 is a prime.

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

It is an Ulam number.

21015 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 20988 and 21006.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21011) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 189 + ... + 278.

It is an arithmetic number, because the mean of its divisors is an integer number (3042).

2²¹⁰¹⁵ is an apocalyptic number.

21015 is a deficient number, since it is larger than the sum of its proper divisors (15489).

21015 is a wasteful number, since it uses less digits than its factorization.

21015 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 478 (or 475 counting only the distinct ones).

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

The square root of 21015 is about 144.9655131402. The cubic root of 21015 is about 27.5958090675.

Adding to 21015 its product of nonzero digits (10), we get a square (21025 = 145²).

Adding to 21015 its reverse (51012), we get a palindrome (72027).

It can be divided in two parts, 210 and 15, that added together give a square (225 = 15²).

The spelling of 21015 in words is "twenty-one thousand, fifteen".