21755581 has 2 divisors, whose sum is σ = 21755582. Its totient is φ = 21755580.

The previous prime is 21755579. The next prime is 21755611. The reversal of 21755581 is 18555712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12355225 + 9400356 = 3515^2 + 3066^2 .

It is a cyclic number.

It is not a de Polignac number, because 21755581 - 2¹ = 21755579 is a prime.

It is a super-2 number, since 2×21755581² = 946610609295122, which contains 22 as substring.

Together with 21755579, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21755381) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10877790 + 10877791.

It is an arithmetic number, because the mean of its divisors is an integer number (10877791).

Almost surely, 2^21755581 is an apocalyptic number.

It is an amenable number.

21755581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21755581 is an equidigital number, since it uses as much as digits as its factorization.

21755581 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 14000, while the sum is 34.

The square root of 21755581 is about 4664.2878341715. The cubic root of 21755581 is about 279.1623819743.

The spelling of 21755581 in words is "twenty-one million, seven hundred fifty-five thousand, five hundred eighty-one".