CHICAGO — The Knicks coaching search is expanding beyond geometry. Phil Jackson, according to an NBA source, is OK hiring a head coach who doesn’t specialize in the triangle.

According to the source, that means Mark Jackson is on Jackson’s tentative list that once included just one name. But now Steve Kerr is headed to Golden State and Jackson is headed on a more expansive search.

The Knicks believe Jackson’s “credibility’’ around the league will enable him to have the “flexibility” to hire an experienced coach willing to adapt to the principles of his triangle offense — or at least a facsimile.

According to a source, Jackson was to meet with Knicks general manager Steve Mills on Thursday night at Chicago’s pre-draft combine to go over some candidates. Jackson stayed away from the gym where players worked out, presumably to avoid media.

Hiring Mark Jackson, the former St. John’s star and Brooklyn native, would be the splashy move for Phil Jackson to save face after Kerr left him at the altar. In a delightful twist, Mark Jackson was fired by Golden State last week to set in motion the Warriors stealing Kerr. Mark Jackson, despite leading the Warriors to their first 50-win season in 20 years, was reportedly fired due to his my-way-or-the-highway approach.

However, under Phil Jackson, that certainly wouldn’t be his mindset, considering the Jacksons between them have 13 rings — all Phil’s.

Phil Jackson also has ex-Lakers point guard Derek Fisher on his radar — considered the next-best thing to Kerr because of his history of running the triangle in Jackson’s offense. A source said he believes a bidding war could open for Fisher, though, between the Lakers and Knicks.

However, that can’t happen for a little while, at least. Fisher, former president of the Players Association, is a backup point guard on Oklahoma City, which advanced to the Western Conference finals Thursday. Fisher conceivably could be playing into late June.

In addition, Fisher said in early April he planned to sit out a year to be with his family before contemplating a coaching or front-office career. “That is not in my radar or in my scope or anything that I’ve thought about, even coaching in general,” Fisher told The Oklahoman last month.

Hence, Jackson could make more sense. He was runner-up to Mike D’Antoni in the Knicks’ last full-fledged coaching search and has privately yearned to coach at the Garden.

In a telling quote before his Garden coaching debut in 2012-13, Jackson said, “These are things I thought about as a kid, I dreamt about as a kid. It’s not a throwaway thing. It’s a ‘wow’ moment. That’s what makes it an emotional thing, not coaching the game, but as a kid: ‘Who woulda thunk it?’”

According to two sources, Jackson is not yet thinking of going back to the bench after the Kerr rejection. His health issues are legitimate despite fiancée Jeanie Buss recently urging Jax to go back to the bench. Not only does Jackson need a second knee replacement surgery eventually, but he’s dealt with prostate cancer and still takes medication for a heart ailment.

Jackson also likely will interview his former assistants, Kurt Rambis and Jim Cleamons, but needs to ask permission from the Lakers and Bucks, respectively. Both are triangle gurus. A source also said Jackson thinks highly of Pacers assistant Nate McMillan, who is still coaching in the playoffs. Brian Shaw told The Denver Post on Thursday he wasn’t interested in interviewing for the Knicks job, as he’s happy with the Nuggets.