21243581 has 2 divisors, whose sum is σ = 21243582. Its totient is φ = 21243580.

The previous prime is 21243533. The next prime is 21243589. The reversal of 21243581 is 18534212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15531481 + 5712100 = 3941^2 + 2390^2 .

It is a cyclic number.

It is not a de Polignac number, because 21243581 - 2⁶ = 21243517 is a prime.

It is a super-2 number, since 2×21243581² = 902579467407122, which contains 22 as substring.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21243581.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21243589) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10621790 + 10621791.

It is an arithmetic number, because the mean of its divisors is an integer number (10621791).

Almost surely, 2^21243581 is an apocalyptic number.

It is an amenable number.

21243581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21243581 is an equidigital number, since it uses as much as digits as its factorization.

21243581 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1920, while the sum is 26.

The square root of 21243581 is about 4609.0759377559. The cubic root of 21243581 is about 276.9550205653.

Adding to 21243581 its reverse (18534212), we get a palindrome (39777793).

It can be divided in two parts, 21 and 243581, that multiplied together give a triangular number (5115201 = T₃₁₉₈).

The spelling of 21243581 in words is "twenty-one million, two hundred forty-three thousand, five hundred eighty-one".