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For n >= 6, max( prevprime(n), 2*prevprime(floor(n/2))).
1

%I #5 Mar 30 2012 16:47:32

%S 1,2,2,3,3,5,5,7,7,7,7,11,11,13,13,14,14,17,17,19,19,19,19,23,23,23,

%T 23,26,26,29,29,31,31,31,31,34,34,37,37,38,38,41,41,43,43,43,43,47,47,

%U 47,47,47,47,53,53,53,53,53

%N For n >= 6, max( prevprime(n), 2*prevprime(floor(n/2))).

%D S. K. Stein and S. Szabo, Algebra and Tiling, MAA Carus Monograph 25, 1994, page 104.

%Y Cf. A035031.

%K nonn

%O 1,2

%A _N. J. A. Sloane_.