21009761 has 2 divisors, whose sum is σ = 21009762. Its totient is φ = 21009760.

The previous prime is 21009749. The next prime is 21009827. The reversal of 21009761 is 16790012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20611600 + 398161 = 4540^2 + 631^2 .

It is a cyclic number.

It is not a de Polignac number, because 21009761 - 2⁶ = 21009697 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21009761.

It is not a weakly prime, because it can be changed into another prime (21009701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504880 + 10504881.

It is an arithmetic number, because the mean of its divisors is an integer number (10504881).

Almost surely, 2^21009761 is an apocalyptic number.

It is an amenable number.

21009761 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009761 is an equidigital number, since it uses as much as digits as its factorization.

21009761 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 756, while the sum is 26.

The square root of 21009761 is about 4583.6405836409. The cubic root of 21009761 is about 275.9351568247.

Adding to 21009761 its reverse (16790012), we get a palindrome (37799773).

The spelling of 21009761 in words is "twenty-one million, nine thousand, seven hundred sixty-one".