21009581 has 2 divisors, whose sum is σ = 21009582. Its totient is φ = 21009580.

The previous prime is 21009577. The next prime is 21009587. The reversal of 21009581 is 18590012.

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., 15760900 + 5248681 = 3970^2 + 2291^2 .

It is a cyclic number.

It is not a de Polignac number, because 21009581 - 2² = 21009577 is a prime.

It is a super-2 number, since 2×21009581² = 882804987591122, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21009581.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21009587) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504790 + 10504791.

It is an arithmetic number, because the mean of its divisors is an integer number (10504791).

Almost surely, 2^21009581 is an apocalyptic number.

It is an amenable number.

21009581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009581 is an equidigital number, since it uses as much as digits as its factorization.

21009581 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 21009581 is about 4583.6209485515. The cubic root of 21009581 is about 275.9343688026.

Adding to 21009581 its reverse (18590012), we get a palindrome (39599593).

The spelling of 21009581 in words is "twenty-one million, nine thousand, five hundred eighty-one".