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A175305 a(1)=1. a(n) = the smallest integer > a(n-1) where both a(n)-1 and a(n)+1 are coprime to all earlier terms of the sequence. 1

%I

%S 1,2,4,6,12,18,24,30,42,60,72,102,108,138,150,168,180,192,198,228,240,

%T 270,282,312,348,420,432,462,522,570,600,618,642,660,810,822,828,858,

%U 882,1020,1032,1050,1062,1092,1152,1230,1278,1290,1302,1320,1368,1428

%N a(1)=1. a(n) = the smallest integer > a(n-1) where both a(n)-1 and a(n)+1 are coprime to all earlier terms of the sequence.

%C Every term from a(4) on is a multiple of 6.

%p A175305 := proc(n) option remember ; local wrks ; if n = 1 then 1; else for a from procname(n-1)+1 do wrks := true; for i from 1 to n-1 do if gcd(a-1,procname(i)) <> 1 or gcd(a+1,procname(i)) <> 1 then wrks := false; end if; end do: if wrks then return a; end if; end do: end if; end proc: seq(A175305(n),n=1..80) ; # _R. J. Mathar_, Apr 15 2010

%K nonn

%O 1,2

%A _Leroy Quet_, Apr 11 2010

%E More terms from _R. J. Mathar_, Apr 15 2010

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Last modified June 19 04:44 EDT 2019. Contains 324217 sequences. (Running on oeis4.)