Solution
= Solution
Translation preserves the alternating on-site term only when $(-1)^{j+\ell}=(-1)^j$, so
$$
[H_t,T_\ell]=0\quad\Longleftrightarrow\quad \ell\in2\mathbb Z.
$$
The primitive lattice spacing is therefore $2a$.
= Solution
Translation preserves the alternating on-site term only when $(-1)^{j+\ell}=(-1)^j$, so
$$
[H_t,T_\ell]=0\quad\Longleftrightarrow\quad \ell\in2\mathbb Z.
$$
The primitive lattice spacing is therefore $2a$.