21809664 has 60 divisors (see below), whose sum is σ = 62957466. Its totient is φ = 7268352.

The previous prime is 21809651. The next prime is 21809677. The reversal of 21809664 is 46690812.

It is an interprime number because it is at equal distance from previous prime (21809651) and next prime (21809677).

It can be written as a sum of positive squares in only one way, i.e., 20793600 + 1016064 = 4560^2 + 1008^2 .

It is a super-2 number, since 2×21809664² = 951322887585792, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is an unprimeable number.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 2242 + ... + 6974.

Almost surely, 2^21809664 is an apocalyptic number.

21809664 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21809664, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31478733).

21809664 is an abundant number, since it is smaller than the sum of its proper divisors (41147802).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21809664 is an equidigital number, since it uses as much as digits as its factorization.

21809664 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4757 (or 4738 counting only the distinct ones).

The product of its (nonzero) digits is 20736, while the sum is 36.

The square root of 21809664 is about 4670.0817979988. The cubic root of 21809664 is about 279.3935172328.

The spelling of 21809664 in words is "twenty-one million, eight hundred nine thousand, six hundred sixty-four".