Background: Here is a group G of a people, one maybe another's friend. How to select least number of people to be a leader of a subgroup, so that everyone in the group G has a friend as a leader?
Translate: find least number of radial-subgraph of a graph. By radial-subgraph, it means a subgraph which has at least one point which connects all the other point in the subgraph.
This is actually Dominating set problem, check in wikipedia https://en.wikipedia.org/wiki/Dominating_set
I think the application is, how to select least number of people in group to tell a message, so that all of the people in the group can get that message via friends.