21075781 has 2 divisors, whose sum is σ = 21075782. Its totient is φ = 21075780.

The previous prime is 21075773. The next prime is 21075809. The reversal of 21075781 is 18757012.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12320100 + 8755681 = 3510^2 + 2959^2 .

It is a cyclic number.

It is not a de Polignac number, because 21075781 - 2³ = 21075773 is a prime.

It is a super-2 number, since 2×21075781² = 888377089519922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21075181) 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 as a sum of consecutive naturals, namely, 10537890 + 10537891.

It is an arithmetic number, because the mean of its divisors is an integer number (10537891).

Almost surely, 2^21075781 is an apocalyptic number.

21075781 is the 2904-th centered pentagonal number.

It is an amenable number.

21075781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21075781 is an equidigital number, since it uses as much as digits as its factorization.

21075781 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3920, while the sum is 31.

The square root of 21075781 is about 4590.8366339917. The cubic root of 21075781 is about 276.2238827912.

The spelling of 21075781 in words is "twenty-one million, seventy-five thousand, seven hundred eighty-one".