21006215 has 4 divisors (see below), whose sum is σ = 25207464. Its totient is φ = 16804968.

The previous prime is 21006163. The next prime is 21006217. The reversal of 21006215 is 51260012.

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 21006215 - 2⁶ = 21006151 is a prime.

It is a super-2 number, since 2×21006215² = 882522137252450, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21006193 and 21006202.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21006217) 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, 2100617 + ... + 2100626.

It is an arithmetic number, because the mean of its divisors is an integer number (6301866).

Almost surely, 2^21006215 is an apocalyptic number.

21006215 is a deficient number, since it is larger than the sum of its proper divisors (4201249).

21006215 is an equidigital number, since it uses as much as digits as its factorization.

21006215 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4201248.

The product of its (nonzero) digits is 120, while the sum is 17.

The square root of 21006215 is about 4583.2537568849. The cubic root of 21006215 is about 275.9196319595.

Adding to 21006215 its reverse (51260012), we get a palindrome (72266227).

The spelling of 21006215 in words is "twenty-one million, six thousand, two hundred fifteen".