21043661 has 2 divisors, whose sum is σ = 21043662. Its totient is φ = 21043660.

The previous prime is 21043613. The next prime is 21043667. The reversal of 21043661 is 16634012.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19272100 + 1771561 = 4390^2 + 1331^2 .

It is a cyclic number.

It is not a de Polignac number, because 21043661 - 2¹⁴ = 21027277 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21043667) 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, 10521830 + 10521831.

It is an arithmetic number, because the mean of its divisors is an integer number (10521831).

Almost surely, 2^21043661 is an apocalyptic number.

It is an amenable number.

21043661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21043661 is an equidigital number, since it uses as much as digits as its factorization.

21043661 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 864, while the sum is 23.

The square root of 21043661 is about 4587.3370270779. The cubic root of 21043661 is about 276.0834874857.

Adding to 21043661 its reverse (16634012), we get a palindrome (37677673).

The spelling of 21043661 in words is "twenty-one million, forty-three thousand, six hundred sixty-one".