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 A258206 Number of strictly non-overlapping holeless polyhexes of perimeter 2n, counted up to rotations and turning over. 15
 0, 0, 1, 0, 1, 1, 3, 2, 12, 14, 50, 97, 312, 744, 2291 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,7 COMMENTS Differs from A057779 for the first time at n=12 as here a(12) = 97, one less than A057779(12) because this sequence excludes polyhexes with holes, the smallest which contains six hexagons in a ring, enclosing a hole of one hex, having thus perimeter of 18+6 = 24 (= 2*12) edges. Differs from A258019 for the first time at n=13 as here a(13) = 312, one less than A258019(13) because this sequence counts only strictly non-overlapping and non-touching polyhex-patterns, while A258019(13) already includes one specimen of helicene-like self-reaching structures. If one counts these structures by the number of hexagons (instead of perimeter length), one obtains sequence 1, 1, 3, 7, 22, 81, ... (A018190). a(n) is also the number of 2n-step 2-dimensional closed self-avoiding paths on honeycomb lattice, reduced for symmetry. - Luca Petrone, Jan 08 2016 LINKS Hugo Pfoertner, Illustration of polygons of perimeter <= 20. FORMULA a(n) = (1/2) * (A258204(n) + A258205(n)). Other observations. For all n >= 1: a(n) <= A057779(n). a(n) <= A258019(n). PROG (Scheme) (define (A258206 n) (* (/ 1 2) (+ (A258204 n) (A258205 n)))) CROSSREFS Cf. A000228, A057779, A018190, A258019, A258204, A258205, A316193. Sequence in context: A195200 A098646 A129925 * A267011 A258019 A057779 Adjacent sequences:  A258203 A258204 A258205 * A258207 A258208 A258209 KEYWORD nonn,walk,more AUTHOR Antti Karttunen, May 31 2015 EXTENSIONS a(14)-a(15) from Luca Petrone, Jan 08 2016 STATUS approved

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Last modified June 16 12:38 EDT 2019. Contains 324152 sequences. (Running on oeis4.)