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%I #13 Dec 10 2015 02:22:15
%S 1,2,3,6,9,8,7,4,12,23,36,69,98,87,74,41,123,236,369,698,987,874,741,
%T 412,1236,2369,3698,6987,9874,8741,7412,4123,12369,23698,36987,69874,
%U 98741,87412,74123,41236,123698,236987,369874,698741,987412,874123,741236,412369
%N Proceed counterclockwise on the outer keys of a numeric keypad (i.e., 1,2,3,6,9,8,7,4): first single digits, then concatenate two digits, then three, etc.
%C k-digit terms start at index 8k-7; for example, at index 57, start the 8-digit terms 12369874, 23698741, 36987412, 69874123, 98741236, 87412369, 74123698, 41236987.
%o (PARI) d=Vec("12369874");for(L=1,6,for(i=1,#d,print1(concat(vector(L,j,d[(i+j-2)%#d+1]))",")))
%Y Cf. A059805, A091331, A091332, A249163.
%K nonn,base
%O 1,2
%A _M. F. Hasler_, Oct 23 2014