21506661 has 48 divisors (see below), whose sum is σ = 36388800. Its totient is φ = 12529440.

The previous prime is 21506659. The next prime is 21506669. The reversal of 21506661 is 16660512.

It is a happy number.

It is not a de Polignac number, because 21506661 - 2¹ = 21506659 is a prime.

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

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

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21506669) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 94630 + ... + 94856.

It is an arithmetic number, because the mean of its divisors is an integer number (758100).

Almost surely, 2^21506661 is an apocalyptic number.

It is an amenable number.

21506661 is a deficient number, since it is larger than the sum of its proper divisors (14882139).

21506661 is a wasteful number, since it uses less digits than its factorization.

21506661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 287 (or 270 counting only the distinct ones).

The product of its (nonzero) digits is 2160, while the sum is 27.

The square root of 21506661 is about 4637.5274662259. The cubic root of 21506661 is about 278.0936014885.

The spelling of 21506661 in words is "twenty-one million, five hundred six thousand, six hundred sixty-one".