21391663 has 8 divisors (see below), whose sum is σ = 22887920. Its totient is φ = 19932480.

The previous prime is 21391651. The next prime is 21391703. The reversal of 21391663 is 36619312.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21391663 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21391603) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 8070 + ... + 10387.

It is an arithmetic number, because the mean of its divisors is an integer number (2860990).

Almost surely, 2^21391663 is an apocalyptic number.

21391663 is a deficient number, since it is larger than the sum of its proper divisors (1496257).

21391663 is a wasteful number, since it uses less digits than its factorization.

21391663 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18537.

The product of its digits is 5832, while the sum is 31.

The square root of 21391663 is about 4625.1122148549. The cubic root of 21391663 is about 277.5970517879.

The spelling of 21391663 in words is "twenty-one million, three hundred ninety-one thousand, six hundred sixty-three".