21663361 has 4 divisors (see below), whose sum is σ = 21673300. Its totient is φ = 21653424.

The previous prime is 21663359. The next prime is 21663373. The reversal of 21663361 is 16336612.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 10400625 + 11262736 = 3225^2 + 3356^2 .

It is a cyclic number.

It is not a de Polignac number, because 21663361 - 2¹ = 21663359 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21663391) 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, 126 + ... + 6583.

It is an arithmetic number, because the mean of its divisors is an integer number (5418325).

Almost surely, 2^21663361 is an apocalyptic number.

It is an amenable number.

21663361 is a deficient number, since it is larger than the sum of its proper divisors (9939).

21663361 is an equidigital number, since it uses as much as digits as its factorization.

21663361 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9938.

The product of its digits is 3888, while the sum is 28.

The square root of 21663361 is about 4654.3915821512. The cubic root of 21663361 is about 278.7673750288.

Adding to 21663361 its reverse (16336612), we get a palindrome (37999973).

The spelling of 21663361 in words is "twenty-one million, six hundred sixty-three thousand, three hundred sixty-one".