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%I #8 Jul 22 2014 20:12:13
%S 7,21,70,245,861,3010,10535,36876,129066,451731,1581055,5533696,
%T 19367936,67787776,237257216,830400256,2906400896,10172403136,
%U 35603410976,124611938416,436141784456,1526496245596,5342736859586,18699579008551,65448526529925,229069842854741
%N Number of nonnegative integers with property that their base 7/2 expansion (see A024639) has n digits.
%e The numbers 7-27 are represented by 20, 21, 22, 23, 24, 25, 26, 40, 41, 42, 43, 44, 45, 46, 60, 61, 62, 63, 64, 65, 66 respectively in base 7/2. These are the only integers with two digits, and so a(2)=21.
%o (Sage)
%o A=[1]
%o for i in [1..60]:
%o A.append(ceil((7-2)/2*sum(A)))
%o [7*x for x in A]
%Y Cf. A024639, A081848.
%K nonn,base
%O 0,1
%A _James Van Alstine_, Jul 21 2014