21661015 has 4 divisors (see below), whose sum is σ = 25993224. Its totient is φ = 17328808.

The previous prime is 21661001. The next prime is 21661027. The reversal of 21661015 is 51016612.

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 21661015 - 2²³ = 13272407 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2166097 + ... + 2166106.

It is an arithmetic number, because the mean of its divisors is an integer number (6498306).

Almost surely, 2^21661015 is an apocalyptic number.

21661015 is a deficient number, since it is larger than the sum of its proper divisors (4332209).

21661015 is an equidigital number, since it uses as much as digits as its factorization.

21661015 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4332208.

The product of its (nonzero) digits is 360, while the sum is 22.

The square root of 21661015 is about 4654.1395552776. The cubic root of 21661015 is about 278.7573117724.

Adding to 21661015 its reverse (51016612), we get a palindrome (72677627).

The spelling of 21661015 in words is "twenty-one million, six hundred sixty-one thousand, fifteen".