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a(n) = 2^(n-1) mod semiprime(n).
2

%I #19 Jun 27 2024 09:01:38

%S 1,2,4,8,2,2,1,18,6,18,1,8,1,22,4,16,23,2,14,2,52,2,49,2,10,65,64,8,

%T 44,2,64,2,42,12,62,56,31,98,81,50,74,2,1,128,96,71,60,7,81,16,94,146,

%U 1,144,108,154,133,4,46,153,67,44,88,90,198,32,14,117

%N a(n) = 2^(n-1) mod semiprime(n).

%H Harvey P. Dale, <a href="/A175131/b175131.txt">Table of n, a(n) for n = 1..1000</a>

%t Module[{nn=300,sems,len},sems=Select[Range[nn],PrimeOmega[#]==2&];len = Length[sems];PowerMod[2,#[[1]]-1,#[[2]]]&/@Thread[{Range[len],sems}]] (* _Harvey P. Dale_, May 19 2018 *)

%o (PARI) lista(nn) = {semip = select(n->bigomega(n) == 2, vector(nn, i, i)); for (i=1, #semip, print1(2^(i-1) % semip[i], ", "););} \\ _Michel Marcus_, Sep 11 2013

%Y Cf. A000079, A001358, A175036, A181615.

%K nonn,less

%O 1,2

%A _Juri-Stepan Gerasimov_, Dec 02 2010

%E More terms from _Michel Marcus_, Sep 11 2013