Assumed first published in the collection End of the Tiger as it's not sourced/credited on the copyright page of the 1966, Fawcett edition. (source: Copyright page for End of the Tiger (1966, Fawcett Publications, #449-02480-075, $0.75, 192pp, pb, coll))
Triangle was simultaneously published in Cavalier magazine (archive.org/details/cavalier-v-16-n-11-1966-09). Which is really the first publication is hard to say.
| Date | Publication | Publisher | Type | Page | |
|---|---|---|---|---|---|
| 1966 |
|
End of the Tiger and Other Stories | Fawcett Gold Medal | Collection | 93 |
| 1969 | End of the Tiger and Other Short Stories | Fawcett Gold Medal | Collection | 93 | |
| 1979 |
|
End of the Tiger and Other Short Stories | Fawcett Gold Medal | Collection | 93 |