Vrijeme: 08:58
fiFA | phiFA #1
Matko se umorio od vježbanja za natjecanje iz matematike pa se odlučio opustiti uz FIFA-u 23. Međutim, kada je upalio računalo (Matko nema para za PlayStation®) dočekalo ga je neugodno iznenađenje. Upravo se izvršava ažuriranje kojem se ne nazire kraja ni konca. Matko je gorko zaplakao i ispustio krik pun boli. Njegov je vapaj dopro do ušiju starijeg mu brata Marina, inače studenta PMF matematike, koji se sažalio nad jadnim Matkom. Marin je znao da se njegov mlađi brat voli zabavljati uz FIFA-u, no zbog previše fakultetskih obaveza on je sam nikad nije igrao. Nije zapravo znao ni što FIFA jest, tko bi uopće popratio sva ta online natjecanja na kojima današnja mladost sudjeluje? Kako bilo, Marin je u želji od oraspoloži Matka na brzinu smislio nekoliko fiFA zadataka.
Odredi zbroj prirodnih brojeva za koje vrijedi 
.
Napomena: ovdje, kao i u ostatku lanca, 
se odnosi na Eulerovu funkciju.
Matko got tired of practicing for the math competition, so he decided to relax with FIFA 23. However, when he turned on his computer (Matko can't afford a PlayStation®), he was met with an unpleasant surprise. An update was running with no end in sight. Matko burst into tears and let out a cry full of pain. His wail reached the ears of his older brother Marin, a math student at PMF, who took pity on poor Matko. Marin knew his younger brother liked to have fun with FIFA, but with too many university commitments, he had never played it himself. He didn’t even really know what FIFA was—who could keep up with all those online competitions the youth participate in these days? Either way, wanting to cheer Matko up, Marin quickly came up with a few FIFA problems.
Determine the sum of natural numbers for which 
.
Note: Here, as in the rest of the sequence, refers to Euler's function.