TITLE: 1, 5, 9, 13, 17, 21,...
QUESTION [8 upvotes]: How would you describe the set $\{1, 5, 9, 13, 17, 21,\dots\}$ in the style of $x:P(x)=$? I know that the sequence is "the last number + 4" or $4n-3$.

REPLY [3 votes]: $\{n\in\mathbb N:\frac{n+3}4\in\mathbb N\}$