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A009293 If a, b in sequence, so is a*b+1.

%I A009293 #18 Jul 23 2018 11:48:35
%S A009293 2,5,11,23,26,47,53,56,95,107,113,116,122,131,191,215,227,233,236,245,
%T A009293 254,263,266,281,287,383,431,455,467,473,476,491,509,518,527,530,533,
%U A009293 536,563,566,575,581,584,599,611,617,656,677,767,863,911,935,947,953,956,983
%N A009293 If a, b in sequence, so is a*b+1.
%H A009293 Alois P. Heinz, <a href="/A009293/b009293.txt">Table of n, a(n) for n = 1..20000</a> (first 1000 terms from Reinhard Zumkeller)
%t A009293 f[l_] := Block[{k = l}, Select[ Union[ Flatten[ AppendTo[k, Table[ k[[i]]*k[[j]] + 1, {i, 1, Length[k]}, {j, 1, i}]]]], # < 1000 &]]; NestList[f, {2}, 7][[ -1]] (* _Robert G. Wilson v_, May 23 2004 *)
%o A009293 (Haskell)
%o A009293 import Data.Set (singleton, deleteFindMin, insert)
%o A009293 a009293 n = a009293_list !! (n-1)
%o A009293 a009293_list = f [2] (singleton 2) where
%o A009293    f xs s = m : f xs' (foldl (flip insert) s' (map ((+ 1) . (* m)) xs'))
%o A009293      where xs' = m : xs
%o A009293            (m,s') = deleteFindMin s
%o A009293 -- _Reinhard Zumkeller_, Aug 15 2011
%Y A009293 Cf. A009388, A009299, A192476.
%K A009293 nonn
%O A009293 1,1
%A A009293 _David W. Wilson_