I have read from the Gallian's abstract algebra textbook the following result
$R[X]$ is a principal ideal ring if and only if $R\cong R_{1}\oplus R_{2}\oplus\cdots \oplus R_{n}$, where each $R_{i}$ is a field.
which the book said it was proved by undergraduates. And I try to prove it on my own, but it resists all my attempts. I try to seek the original paper on Internet, but I haven't got a clue. I am eager to know how to prove it. Any help or hint will be appreciated, thank you so much!