This post is part of the Genuinely Approachable Sudoku (GAS) series.
It might take a little bit of time to understand all the column indexing going on here, but it always involves finding possibly linked cells, one of them in red in column 1, 5, or 9, and the other being the place for the 1, 5, or 9 itself.

PDF

or solve online (using SudokuPad)

Theme: A Little from Column 159

Author/Opus: This is the 13th puzzle from Philip Newman, part of the Genuinely Approachable Sudoku (GAS) team.

Rules: Standard Sudoku rules (Insert a number from 1 to 9 into each cell so that no number repeats in any row, column, or bold region). Also, the cells marked in red are column indexers. In this puzzle, 1, 5, and 9 are the column indexers. If the number X is in a red cell in column Y in a given row, then in the same row the number Y is in column X. (For example, if in the top row there is a 4 in column 1 in the red column indexer, then the 1 in that row is in column 4; similarly, if there is a 5 in column 7 of the top row, that would mean column 5 of the top row contains a 7 because of where the 5 would be indexed.)

GAS Time Standards (highlight to view): Two party hats (🎩🥳): 6:30; One party hat (🥳): 12:00. All other solvers earn a 🦕: Caryatidic Chubutisaurid.

Thomas Hits the GAS (highlight to view): 2:12, with SudokuPad replay file shared as a download for now (requires loading via settings menu with improvements expected before people should use this regularly).

Solution: PDF; solving video with explanation to be added later from GAS team.

Note: Follow this link for other less common variations of Sudoku.

Note 2: Comments on the blog are great! For a more interactive discussion, please also consider using our Daily GAS discussion post on the GMPuzzles Discord. Not a member of the Discord? Click this link for basic access.