Not long after I learned about Frozen, I went to see a friend of mine who works in the music industry. We sat in his living room on the Upper East Side, facing each other in easy chairs, as he worked his way through a mountain of CDs. He played “Angel,” by the reggae singer Shaggy, and then “The Joker,” by the Steve Miller Band, and told me to listen very carefully to the similarity in bass lines. He played Led Zeppelin’s “Whole Lotta Love” and then Muddy Waters’s “You Need Love,” to show the extent to which Led Zeppelin had mined the blues for inspiration. He played “Twice My Age,” by Shabba Ranks and Krystal, and then the saccharine ’70s pop standard “Seasons in the Sun,” until I could hear the echoes of the second song in the first. He played “Last Christmas,” by Wham! followed by Barry Manilow’s “Can’t Smile Without You” to explain why Manilow might have been startled when he first heard that song, and then “Joanna,” by Kool and the Gang, because, in a different way, “Last Christmas” was an homage to Kool and the Gang as well. “That sound you hear in Nirvana,” my friend said at one point, “that soft and then loud kind of exploding thing, a lot of that was inspired by the Pixies. Yet Kurt Cobain” — Nirvana’s lead singer and songwriter — “was such a genius that he managed to make it his own. And ‘Smells Like Teen Spirit’?” — here he was referring to perhaps the best-known Nirvana song. “That’s Boston’s ‘More Than a Feeling.’ ” He began to hum the riff of the Boston hit, and said, “The first time I heard ‘Teen Spirit,’ I said, ‘That guitar lick is from “More Than a Feeling.” ’ But it was different — it was urgent and brilliant and new.” He played another CD. It was Rod Stewart’s “Do Ya Think I’m Sexy,” a huge hit from the 1970s. The chorus has a distinctive, catchy hook — the kind of tune that millions of Americans probably hummed in the shower the year it came out. Then he put on “Taj Mahal,” by the Brazilian artist Jorge Ben Jor, which was recorded several years before the Rod Stewart song. In his twenties, my friend was a DJ at various downtown clubs, and at some point he’d become interested in world music. “I caught it back then,” he said. A small, sly smile spread across his face. The opening bars of “Taj Mahal” were very South American, a world away from what we had just listened to. And then I heard it. It was so obvious and unambiguous that I laughed out loud; virtually note for note, it was the hook from “Do Ya Think I’m Sexy.” It was possible that Rod Stewart had independently come up with that riff, because resemblance is not proof of influence. It was also possible that he’d been in Brazil, listened to some local music, and liked what he heard.