<s>[INST] Binomial coefficients are odd if and only if $n = 2^k-1$ 
Let $n$ be an integer, $n > 0$. Prove that all the coefficients of the expansion of the Newtonian binomial $(a+b)^n$ are odd if and only if $n$ is of the form $2^k-1$.

Why in the solution below do they take $n > 8$? Also, I don't get the middle paragraph argument about $n_1$. What do those values represent using $n_1$?
Solution:
For $n \leq 8$ the theorem is immediately verifiable. It is therefore sufficient to assume that the theorem is true for the binomials $a+b,(a+b)^2,\ldots,(a+b)^{n-1}$ for $n > 8$, and prove that the theorem holds for $(a+b)^n$. The coefficients of the binomial expansion (except for the extreme ones both equal to $1$) are $$\dfrac{n}{1}, \dfrac{n(n-1)}{1 \cdot 2},\ldots,\dfrac{n(n-1) \cdots 1}{1 \cdot 2 \cdots (n-1)}.$$ A necessary and sufficient condition in order that all these numbers be odd is that $n$ is odd and the numbers obtained by deleting the odd factors from the numerators and denominators of the remaining numbers be odd.
But setting $n = 2n_1+1$ these numbers can be represented by the terms of the sequence $$\dfrac{n_1}{1},\dfrac{n_1(n_1-1)}{1 \cdot 2}, \ldots, \dfrac{n_1(n_1-1) \cdots 1}{1 \cdot 2 \cdots (n_1-1)}.$$ Since $n_1 < n$, the latter are all odd if and only if $n_1$ is of the form $2^k-1$, i.e. if and only if $n$ is of the form $2(2^k-1)+1 = 2^{k+1}-1$.
 [/INST] If $n=2n_1+1$, then the coefficients are:
$$\frac{2n_1+1}{1}, \frac{(2n_1+1)\cdot (2n_1)}{1\cdot 2}, \cdots , \frac{(2n_1+1)\cdot (2n_1)\cdots 1}{1\cdots (2n_1)}. $$
Removing all the odd factors from the numerator and denomiator we get the sequence:
$$ 1, \frac{2n_1}{2}, \frac{2n_1}{2},\frac{(2n_1)\cdot(2n_1-2)}{2\cdot 4},\frac{(2n_1)\cdot(2n_1-2)}{2\cdot 4},\cdots ,\frac{(2n_1)\cdot(2n_1-2)\cdots 2}{2\cdot 4\cdots 2n_1}. $$Dividing through we get that these are equal to 
$$ 1, \frac{n_1}{1}, \frac{n_1}{1},\frac{(n_1)\cdot(n_1-1)}{1\cdot 2},\frac{(n_1)\cdot(n_1-1)}{1\cdot 2},\cdots ,\frac{(n_1)\cdot(n_1-1)\cdots 1}{1\cdot 2\cdots n_1}. $$These however are just the coefficients in $(a+b)^{n_1}$ repeated twice. Hence by an induction argument we can conclude that these are odd and hence the coefficients in $(a+b)^n$ are odd.
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