21651515 has 4 divisors (see below), whose sum is σ = 25981824. Its totient is φ = 17321208.

The previous prime is 21651499. The next prime is 21651517. The reversal of 21651515 is 51515612.

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 21651515 - 2⁴ = 21651499 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21651517) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2165147 + ... + 2165156.

It is an arithmetic number, because the mean of its divisors is an integer number (6495456).

Almost surely, 2^21651515 is an apocalyptic number.

21651515 is a deficient number, since it is larger than the sum of its proper divisors (4330309).

21651515 is an equidigital number, since it uses as much as digits as its factorization.

21651515 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4330308.

The product of its digits is 1500, while the sum is 26.

The square root of 21651515 is about 4653.1188465372. The cubic root of 21651515 is about 278.7165537300.

The spelling of 21651515 in words is "twenty-one million, six hundred fifty-one thousand, five hundred fifteen".