21663261 has 32 divisors (see below), whose sum is σ = 33744000. Its totient is φ = 13716864.

The previous prime is 21663251. The next prime is 21663277. The reversal of 21663261 is 16236612.

21663261 is a `hidden beast` number, since 21 + 6 + 632 + 6 + 1 = 666.

It is not a de Polignac number, because 21663261 - 2⁹ = 21662749 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21663251) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 56970 + ... + 57348.

It is an arithmetic number, because the mean of its divisors is an integer number (1054500).

Almost surely, 2^21663261 is an apocalyptic number.

It is an amenable number.

21663261 is a deficient number, since it is larger than the sum of its proper divisors (12080739).

21663261 is a wasteful number, since it uses less digits than its factorization.

21663261 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 490 (or 484 counting only the distinct ones).

The product of its digits is 2592, while the sum is 27.

The square root of 21663261 is about 4654.3808395962. The cubic root of 21663261 is about 278.7669460898.

Adding to 21663261 its reverse (16236612), we get a palindrome (37899873).

The spelling of 21663261 in words is "twenty-one million, six hundred sixty-three thousand, two hundred sixty-one".