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%I #27 Mar 17 2022 07:07:29
%S 1,3,9,15,81,123,279,945,1419,3195,7191,24273,36411,81927,276507,
%T 622143,7086609,10629915,23917311,612974547,1379192733,2068789101,
%U 3103183653,4654775481,6982163223,23564800881,35347201323,79531202979,178945206705,268417810059
%N a(n) is the result of factoring a(n-1) + 1 into primes, replacing each prime 2 with a 3, and taking the product of the resulting factors.
%e The term after 123 is 279 because 123 + 1 = 124 = 31*2*2 and 31*3*3 = 279.
%t r23[n_]:=Module[{f=FactorInteger[n+1]},Times@@Flatten[Table[ #[[1]], #[[2]]]&/@ Transpose[Join[{f[[All,1]]/.(2->3),f[[All,2]]}]]]]; NestList[r23,1,30] (* _Harvey P. Dale_, Dec 11 2018 *)
%o (PARI) a=vector(30); a[1]=1; for(n=2, #a, f=factor(a[n-1]+1); if(f[1,1]==2, f[1,1]=3); a[n]=factorback(f)); a \\ _Colin Barker_, May 15 2014
%o (PARI) lista(nn) = {a = 1; for (n=1, nn, print1(a, ", "); a++; nd = valuation(a, 2); a *= 3^nd/2^nd;);} \\ _Michel Marcus_, May 15 2014
%K nonn
%O 1,2
%A _J. Lowell_, May 14 2014
%E More terms from _Colin Barker_, May 14 2014
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