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A243436
Numbers n such that n^2-n-1 is semiprime.
2
8, 13, 15, 18, 19, 23, 24, 26, 28, 30, 33, 34, 35, 38, 41, 44, 50, 52, 58, 59, 62, 64, 68, 70, 72, 73, 74, 75, 76, 78, 79, 80, 82, 83, 88, 89, 91, 92, 96, 98, 99, 100, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 117, 119, 120, 122, 123, 124, 125, 128, 130
OFFSET
1,1
LINKS
EXAMPLE
13 is in the sequence because 13^2 - 13 - 1 = 155 = 5 * 31 is semiprime.
18 is in the sequence because 18^2 - 18 - 1 = 305 = 5 * 61 is semiprime.
MAPLE
with(numtheory):A243436 := proc() if bigomega(n^2-n-1)=2 then RETURN (n); fi; end: seq(A243436 (), n=1..200);
MATHEMATICA
c = 0; Do[If[PrimeOmega[n^2-n-1] == 2, c++; Print[c, " ", n]], {n, 1, 30000}];
Select[Range[200], PrimeOmega[#^2-#-1]==2&] (* Harvey P. Dale, Sep 21 2016 *)
CROSSREFS
Sequence in context: A168137 A252458 A271720 * A214412 A080361 A054295
KEYWORD
nonn
AUTHOR
K. D. Bajpai, Jun 06 2014
STATUS
approved