prime numbers
... that prime numbers have been studied since the time of the ancient Greeks, but had few real-world applications until the invention of public-key cryptography in the 1970s?
... that <b id="mwBpc"><a href="./Prime_number" id="mwBpg" rel="mw:WikiLink" title="Prime number">prime numbers</a></b> have been studied since the time of the ancient Greeks, but had few real-world applications until the invention of <a href="./Public-key_cryptography" id="mwBpk" rel="mw:WikiLink" title="Public-key cryptography">public-key cryptography</a> in the 1970s?