Official rule text
90.12 When researching technology, a player must satisfy each of a technology’s prerequisites to research it. To satisfy a technology’s prerequisites, that player must own one technology of the matching color for each prerequisite symbol on the technology card they wish to research. a Prerequisites symbols are displayed as symbols on the lower-left corner of the card. b Unit upgrade technologies do not have a color and do not satisfy prerequisites. c Players may use certain abilities or technology specialties to ignore some prerequisites.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 90.12