21788681 has 2 divisors, whose sum is σ = 21788682. Its totient is φ = 21788680.

The previous prime is 21788677. The next prime is 21788719. The reversal of 21788681 is 18688712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21529600 + 259081 = 4640^2 + 509^2 .

It is a cyclic number.

It is not a de Polignac number, because 21788681 - 2² = 21788677 is a prime.

It is a super-2 number, since 2×21788681² = 949493239439522, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21788581) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10894340 + 10894341.

It is an arithmetic number, because the mean of its divisors is an integer number (10894341).

Almost surely, 2^21788681 is an apocalyptic number.

It is an amenable number.

21788681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21788681 is an equidigital number, since it uses as much as digits as its factorization.

21788681 is an evil number, because the sum of its binary digits is even.

The product of its digits is 43008, while the sum is 41.

The square root of 21788681 is about 4667.8347228667. The cubic root of 21788681 is about 279.3038873134.

It can be divided in two parts, 21788 and 681, that multiplied together give a triangular number (14837628 = T₅₄₄₇).

The spelling of 21788681 in words is "twenty-one million, seven hundred eighty-eight thousand, six hundred eighty-one".