Search: id:A140135
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%I A140135
%S A140135 16,24,36,54,54,81,100,140,140,140,196,225,315,315,315,315,315,441,484,
%T A140135 550,550,625,676,858,858,858,858,858,858,1089,1156,1225,1330,1330,1444,
%U A140135 1521,1794,1794,1794,1794,1794,1794,2116,2254,2254,2401,2499,2601,2805
%N A140135 Product of largest semiprime <= n and smallest semiprime >= n.
%C A140135 This is to A030664 as semiprimes A001358 are to primes A000040. Subset 
               of A014613.
%F A140135 a(n) = MAX{j in A001358 and j <= n} * MIN{j in A001358 and j >= n}
%e A140135 a(10) = 100 because the largest semiprime <= 10 is 10, the smallest semiprime 
               >= 10 is 10 and 10*10=100.
%p A140135 isA001358 := proc(n) RETURN( numtheory[bigomega](n) = 2) ; end: A001358 
               := proc(n) option remember ; local a; if n = 1 then 4; else for a 
               from A001358(n-1)+1 do if isA001358(a) then RETURN(a) ; fi ; od: 
               fi ; end: prevsemiprime := proc(n) local a; for a from n to 4 by 
               -1 do if isA001358(a) then RETURN(a) ; fi ; od: RETURN(-1) ; end: 
               nextsemiprime := proc(n) local a; for a from n do if isA001358(a) 
               then RETURN(a) ; fi ; od: RETURN(-1) ; end: A140135 := proc(n) prevsemiprime(n)*nextsemiprime(n) 
               ; end: seq(A140135(n),n=4..80) ; - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), 
               May 11 2008
%Y A140135 Cf. A001358, A014613, A030664.
%Y A140135 Sequence in context: A014613 A046370 A103248 this_sequence A120142 A110228 
               A045875
%Y A140135 Adjacent sequences: A140132 A140133 A140134 this_sequence A140136 A140137 
               A140138
%K A140135 easy,nonn
%O A140135 4,1
%A A140135 Jonathan Vos Post (jvospost3(AT)gmail.com), May 09 2008
%E A140135 More terms from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), May 11 2008

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