21765681 has 12 divisors (see below), whose sum is σ = 35930752. Its totient is φ = 12437496.

The previous prime is 21765647. The next prime is 21765683. The reversal of 21765681 is 18656712.

21765681 is a `hidden beast` number, since 21 + 76 + 568 + 1 = 666.

It is not a de Polignac number, because 21765681 - 2¹⁵ = 21732913 is a prime.

It is a super-2 number, since 2×21765681² = 947489738787522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21765683) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 172681 + ... + 172806.

Almost surely, 2^21765681 is an apocalyptic number.

21765681 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21765681 is a deficient number, since it is larger than the sum of its proper divisors (14165071).

21765681 is a wasteful number, since it uses less digits than its factorization.

21765681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 345500 (or 345497 counting only the distinct ones).

The product of its digits is 20160, while the sum is 36.

The square root of 21765681 is about 4665.3704033013. The cubic root of 21765681 is about 279.2055755476.

The spelling of 21765681 in words is "twenty-one million, seven hundred sixty-five thousand, six hundred eighty-one".