This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A075742 #13 Mar 05 2025 22:05:15 %S A075742 2,5,13,89,233,1597,28657,514229,24157817,433494437,2971215073, %T A075742 44945570212853,190392490709135,99194853094755497, %U A075742 1500520536206896083277,3928413764606871165730,1066340417491710595814572169,19134702400093278081449423917 %N A075742 Fibonacci numbers for which both the value and index are the product of an odd number of distinct primes; that is, numbers Fibonacci(k) for which mu(k) = mu(Fibonacci(k)) = -1. %e A075742 11 is a prime and Fibonacci(11) = 89 is a prime, 13 is a prime and Fibonacci(13) = 233 is a prime, but Fibonacci(16) = 987 = 3*7*47 and 16 is not squarefree and 30 = 2*3*5 is the product of an odd number of distinct primes but Fibonacci(30) = 832040 = 2^3*5*11*31*61 is not squarefree, ... %p A075742 with(combinat, fibonacci): m2_supM_fib := proc(n); if (numtheory[mobius](n)=-1) then if (numtheory[mobius](fibonacci(n))=-1) then RETURN(fibonacci(n)); fi; fi; end: %Y A075742 Cf. A000045, A008683, A030059, A074691. %K A075742 easy,nonn %O A075742 1,1 %A A075742 _Jani Melik_, Oct 07 2002 %E A075742 Incorrect 83621143489848422977 removed and more terms from _Sean A. Irvine_, Mar 05 2025