21108781 has 16 divisors (see below), whose sum is σ = 23639040. Its totient is φ = 18753280.

The previous prime is 21108757. The next prime is 21108793. The reversal of 21108781 is 18780112.

It is a cyclic number.

It is not a de Polignac number, because 21108781 - 2⁵ = 21108749 is a prime.

It is a super-2 number, since 2×21108781² = 891161270611922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21108701) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 22716 + ... + 23626.

It is an arithmetic number, because the mean of its divisors is an integer number (1477440).

Almost surely, 2^21108781 is an apocalyptic number.

It is an amenable number.

21108781 is a deficient number, since it is larger than the sum of its proper divisors (2530259).

21108781 is a wasteful number, since it uses less digits than its factorization.

21108781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1004.

The product of its (nonzero) digits is 896, while the sum is 28.

The square root of 21108781 is about 4594.4293443256. The cubic root of 21108781 is about 276.3679760628.

Subtracting from 21108781 its sum of digits (28), we obtain a triangular number (21108753 = T₆₄₉₇).

Adding to 21108781 its reverse (18780112), we get a palindrome (39888893).

The spelling of 21108781 in words is "twenty-one million, one hundred eight thousand, seven hundred eighty-one".