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%I #26 Mar 29 2022 08:31:50
%S 0,1,11,69,96,101,111,609,619,906,916,1001,1111,1691,1961,6009,6119,
%T 6699,6969,9006,9116,9696,9966,10001,10101,11011,11111,16091,16191,
%U 19061,19161,60009,60109,61019,61119,66099,66199,69069,69169,90006,90106,91016,91116
%N Numbers that are the same upside down (using only digits 0, 1, 6 and 9).
%C A000787 without using digit 8, considered here as composed of two circles with different radius. 'Same upside down' means central symmetric, that is 180-degree rotationally symmetric about a central axis perpendicular to the screen plane. See the comment by _M. F. Hasler_ in A000787. - _Wolfdieter Lang_, Oct 25 2013
%H Michael S. Branicky, <a href="/A169731/b169731.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1535 from T. D. Noe)
%o (Python)
%o from itertools import count, islice, product
%o def ud(s): return s[::-1].translate({ord('6'):ord('9'), ord('9'):ord('6')})
%o def agen():
%o yield from [0, 1]
%o for d in count(2):
%o for start in "169":
%o for rest in product("0169", repeat=d//2-1):
%o left = start + "".join(rest)
%o right = ud(left)
%o for mid in [[""], ["0", "1"]][d%2]:
%o yield int(left + mid + right)
%o print(list(islice(agen(), 43))) # _Michael S. Branicky_, Mar 29 2022
%Y Cf. A000787.
%K nonn,base,easy
%O 1,3
%A _N. J. A. Sloane_, May 01 2010, based on a suggestion from Terry Stickels
%E Extended by _T. D. Noe_, May 03 2010