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a(1) = 4, a(n+1) is the largest composite number < 2a(n).
1

%I #6 Dec 05 2013 19:55:48

%S 4,6,10,18,35,69,136,270,539,1077,2152,4303,8605,17208,34415,68829,

%T 137657,275313,550625,1101249,2202497,4404993,8809985,17619969,

%U 35239936,70479871,140959741,281919481,563838961,1127677921,2255355841

%N a(1) = 4, a(n+1) is the largest composite number < 2a(n).

%p a[1] := 4:for n from 2 to 84 do q := 2*a[n-1]-1:while(isprime(q)) do q := q-1:od: a[n] := q:od:seq(a[l],l=1..84);

%Y Cf. A006992, A076994.

%K nonn

%O 1,1

%A _Amarnath Murthy_, Oct 26 2002

%E Corrected and extended by _Sascha Kurz_, Jan 26 2003