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%I #32 May 19 2026 22:01:23
%S 1,4,15,99,630,4599,34099,261603,2036026,16075980,128214840,1031495768
%N Number of free polyforms made of T-pipes connected end-to-end (not perpendicular).
%C Equivalently, a(n) is the number of free polyominoes consisting of n square cells, each of which has one chipped edge, preventing that edge from connecting to another cell. See linked illustrations. - _Pontus von Brömssen_, Nov 12 2025
%H Pontus von Brömssen, <a href="/A390515/a390515.png">The 4 polyominoes with 2 cells, each of which has one chipped edge</a>.
%H Pontus von Brömssen, <a href="/A390515/a390515_1.png">The 15 polyominoes with 3 cells, each of which has one chipped edge</a>.
%H Pontus von Brömssen, <a href="/A390515/a390515_2.png">The 99 polyominoes with 4 cells, each of which has one chipped edge</a>.
%H Pontus von Brömssen, <a href="/A390515/a390515.svg">Alternative illustration for 4 cells</a>.
%H Rhys Feltman, <a href="https://polypad.amplify.com/p/MpK5NKodIbVuqw">Polytees - Polypad</a>.
%H Rhys Feltman, <a href="https://rhys-feltman.neocities.org/polytees">Polytees - Rhys' Polyform Atlas</a>.
%H <a href="/index/Pol#polyominoes">Index entries for sequences related to polyominoes</a>.
%Y Cf. A000012 (I-pipes), A000105 (X-pipes), A000577 (Y-pipes with three 120-degree angles).
%K nonn,more
%O 1,2
%A _Rhys Feltman_, Nov 08 2025
%E a(4)-a(9) from _Pontus von Brömssen_, Nov 13 2025
%E a(10)-a(12) from _Christian Sievers_, Nov 19 2025