What you’d ideally want to find, then, is the smallest subgroup of your friends that knows all the rest of your social circle—which would let you lick the fewest envelopes and still get everyone to attend. Granted, this might sound like a lot of work just to save a few bucks on stamps, but it’s exactly the kind of problem that political campaign managers and corporate marketers want to solve to spread their message most effectively. And it’s also the problem that epidemiologists study in thinking about, say, the minimum number of people in a population—and which people—to vaccinate to protect
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