21331665 has 12 divisors (see below), whose sum is σ = 36974964. Its totient is φ = 11376864.

The previous prime is 21331663. The next prime is 21331669. The reversal of 21331665 is 56613312.

It can be written as a sum of positive squares in 2 ways, for example, as 13927824 + 7403841 = 3732^2 + 2721^2 .

It is not a de Polignac number, because 21331665 - 2¹ = 21331663 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21331663) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 236974 + ... + 237063.

It is an arithmetic number, because the mean of its divisors is an integer number (3081247).

Almost surely, 2^21331665 is an apocalyptic number.

It is an amenable number.

21331665 is a deficient number, since it is larger than the sum of its proper divisors (15643299).

21331665 is a wasteful number, since it uses less digits than its factorization.

21331665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 474048 (or 474045 counting only the distinct ones).

The product of its digits is 3240, while the sum is 27.

The square root of 21331665 is about 4618.6215476049. The cubic root of 21331665 is about 277.3372798160.

Adding to 21331665 its reverse (56613312), we get a palindrome (77944977).

It can be divided in two parts, 213 and 31665, that added together give a triangular number (31878 = T₂₅₂).

The spelling of 21331665 in words is "twenty-one million, three hundred thirty-one thousand, six hundred sixty-five".