21651663 has 16 divisors (see below), whose sum is σ = 31940352. Its totient is φ = 12936000.

The previous prime is 21651607. The next prime is 21651667. The reversal of 21651663 is 36615612.

It is not a de Polignac number, because 21651663 - 2¹² = 21647567 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21651663.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21651667) 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, 2278 + ... + 6963.

It is an arithmetic number, because the mean of its divisors is an integer number (1996272).

Almost surely, 2^21651663 is an apocalyptic number.

21651663 is a deficient number, since it is larger than the sum of its proper divisors (10288689).

21651663 is a wasteful number, since it uses less digits than its factorization.

21651663 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9326.

The product of its digits is 6480, while the sum is 30.

The square root of 21651663 is about 4653.1347498219. The cubic root of 21651663 is about 278.7171887888.

It can be divided in two parts, 2165 and 1663, that added together give a triangular number (3828 = T₈₇).

The spelling of 21651663 in words is "twenty-one million, six hundred fifty-one thousand, six hundred sixty-three".