21903781 has 2 divisors, whose sum is σ = 21903782. Its totient is φ = 21903780.

The previous prime is 21903779. The next prime is 21903793. The reversal of 21903781 is 18730912.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16769025 + 5134756 = 4095^2 + 2266^2 .

It is a cyclic number.

It is not a de Polignac number, because 21903781 - 2¹ = 21903779 is a prime.

It is a super-2 number, since 2×21903781² = 959551244191922, which contains 22 as substring.

Together with 21903779, it forms a pair of twin primes.

It is equal to p_1383529 and since 21903781 and 1383529 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21904781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10951890 + 10951891.

It is an arithmetic number, because the mean of its divisors is an integer number (10951891).

Almost surely, 2^21903781 is an apocalyptic number.

It is an amenable number.

21903781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21903781 is an equidigital number, since it uses as much as digits as its factorization.

21903781 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3024, while the sum is 31.

The square root of 21903781 is about 4680.1475404094. The cubic root of 21903781 is about 279.7948369535.

The spelling of 21903781 in words is "twenty-one million, nine hundred three thousand, seven hundred eighty-one".