Pääkuorilukuina
Pääkuorilukuina (literally “as primary shell numbers”) is a term found in Finnish mathematical literature describing a method of classifying natural numbers by a hierarchical shell decomposition. In this framework, integers are grouped into successive shells, with the outermost group called the head shell. A number is referred to as a pääkuoriluku in a given decomposition if it belongs to the head shell or if it serves as a defining representative for that shell.
Because the precise definition of shells and the rules for moving from one shell to the next
Use and scope: The notion is primarily found in didactic contexts, puzzles, and historical discussions of number
See also: Number classification, Shell decomposition, Layered structures in mathematics, Finnish mathematical terminology.