# Math Help - Canonical Decomp

1. ## Canonical Decomp

$2^{27}+1=(2^9)^3+1^3=(2^9+1)(2^{18}-2^9+1)$ $=(2^3+1)(2^6-2^3+1)(2^{18}-2^9+1)$

What next?

2. It looks like you can write $2^3+1=(2+1)(2^2-2+1)$.

Is this what you are aiming for?

3. No, this section:
$(2^6-2^3+1)(2^{18}-2^9+1)$

4. As polynomials, you can't factor those anymore.

5. I have broken it down to $3^3*19*(2^9*7*73+1)$ but it isn't fully decomp.

What to do next?

6. $2^n$ is congruent to $\pm1$ mod 3, according as n is even or odd. So $2^{18}-2^9+1$ is a multiple of 3. In fact it is equal to 3*87211, and 87211 is prime, as you can check here.