%I A121056
%S A121056 7,9,12,16,24,36,56,90
%N A121056 From a puzzle of Roger Penrose's in the Twistor Newsletter.
%C A121056 Roger Penrose posed the problem of finding the missing term in the sequence 
               "..., 7, 9, 12, ?, 24, 36, 56, 90, ...".
%C A121056 The answer is that the sequence is given by a(n) = 24*(2^n-1)/n, for 
               n = -3, -2, ..., 3, 4 and so the missing entry is (by l'Hopital's 
               rule) 24 log 2 = 16.6355323... [This has been replaced by 16 here 
               to get an integer sequence.]
%C A121056 The sequence for n = -10, ..., 10 is 3069/1280, 511/192, 765/256, 381/
               112, 63/16, 93/20, 45/8, 7, 9, 12, 24 log 2, 24, 36, 56, 90, 744/
               5, 252, 3048/7, 765, 4088/3, 12276/5.
%D A121056 Roger Penrose, Twistor Newsletter, No. 10 (1980), p. 22.
%Y A121056 Cf. A003138, A003176, A003177.
%Y A121056 Sequence in context: A075335 A020720 A048589 this_sequence A112529 A161892 
               A056528
%Y A121056 Adjacent sequences: A121053 A121054 A121055 this_sequence A121057 A121058 
               A121059
%K A121056 nonn
%O A121056 -3,1
%A A121056 N. J. A. Sloane (njas(AT)research.att.com), Aug 10 2006