21131781 has 16 divisors (see below), whose sum is σ = 32355840. Its totient is φ = 12132720.

The previous prime is 21131771. The next prime is 21131797. The reversal of 21131781 is 18713112.

It is a happy number.

It is not a de Polignac number, because 21131781 - 2⁷ = 21131653 is a prime.

It is a super-2 number, since 2×21131781² = 893104336463922, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21131701) 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, 16225 + ... + 17478.

It is an arithmetic number, because the mean of its divisors is an integer number (2022240).

Almost surely, 2^21131781 is an apocalyptic number.

It is an amenable number.

21131781 is a deficient number, since it is larger than the sum of its proper divisors (11224059).

21131781 is a wasteful number, since it uses less digits than its factorization.

21131781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 33736.

The product of its digits is 336, while the sum is 24.

The square root of 21131781 is about 4596.9316940760. The cubic root of 21131781 is about 276.4683159197.

Adding to 21131781 its reverse (18713112), we get a palindrome (39844893).

It can be divided in two parts, 21131 and 781, that added together give a palindrome (21912).

The spelling of 21131781 in words is "twenty-one million, one hundred thirty-one thousand, seven hundred eighty-one".