21331661 has 16 divisors (see below), whose sum is σ = 24561600. Its totient is φ = 18361728.

The previous prime is 21331657. The next prime is 21331663. The reversal of 21331661 is 16613312.

It is a happy number.

It is a cyclic number.

It is not a de Polignac number, because 21331661 - 2² = 21331657 is a prime.

It is a super-2 number, since 2×21331661² = 910079522037842, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

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 15 ways as a sum of consecutive naturals, for example, 15905 + ... + 17193.

It is an arithmetic number, because the mean of its divisors is an integer number (1535100).

Almost surely, 2^21331661 is an apocalyptic number.

It is an amenable number.

21331661 is a deficient number, since it is larger than the sum of its proper divisors (3229939).

21331661 is a wasteful number, since it uses less digits than its factorization.

21331661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1388.

The product of its digits is 648, while the sum is 23.

The square root of 21331661 is about 4618.6211145752. The cubic root of 21331661 is about 277.3372624811.

Adding to 21331661 its reverse (16613312), we get a palindrome (37944973).

The spelling of 21331661 in words is "twenty-one million, three hundred thirty-one thousand, six hundred sixty-one".