Christopher Columbus, during his first voyage to the Americas, probably used a world map created by which German cartographer?

Henricus Martellus was a German cartographer working in Florence in the 15th century. Little is known about his early life, and similarly the contemporary relevance of his work to cartography and exploration is difficult to establish.

There is indirect evidence that Columbus had studied one of his maps in particular, which has been studied at Yale. Columbus wrote about expecting to find Japan in the location and orientation Martellus had depicted it, which was different to all other surviving maps from that time. Furthermore, one of Columbus' crewmembers also described certain islands found in the new world in a way that corresponds to Martellus' map.

At the very least, it's thought that Martellus' maps or similar maps influenced Columbus' geographical perception of Asia's proximity to the west coasts of Africa and Europe that inspired his expedition.

https://news.yale.edu/2015/06/11/hidden-secrets-yale-s-1491-world-map-revealed-multispectral-imaging
https://www.wired.com/2014/09/martellus-map/
https://en.wikipedia.org/wiki/Henricus_Martellus_Germanus