21605766 has 48 divisors (see below), whose sum is σ = 54142704. Its totient is φ = 5697216.

The previous prime is 21605747. The next prime is 21605777. The reversal of 21605766 is 66750612.

It is a Curzon number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 996 + ... + 6648.

It is an arithmetic number, because the mean of its divisors is an integer number (1127973).

Almost surely, 2^21605766 is an apocalyptic number.

21605766 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

It is a practical number, because each smaller number is the sum of distinct divisors of 21605766, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27071352).

21605766 is an abundant number, since it is smaller than the sum of its proper divisors (32536938).

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

21605766 is a wasteful number, since it uses less digits than its factorization.

21605766 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5685 (or 5678 counting only the distinct ones).

The product of its (nonzero) digits is 15120, while the sum is 33.

The square root of 21605766 is about 4648.2002968891. The cubic root of 21605766 is about 278.5201087391.

The spelling of 21605766 in words is "twenty-one million, six hundred five thousand, seven hundred sixty-six".