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 A334556 Numbers whose binary expansion generates rotationally symmetric XOR-triangles. 23
 0, 1, 11, 13, 39, 57, 83, 91, 101, 109, 151, 233, 543, 599, 659, 731, 805, 877, 937, 993, 1379, 1483, 1589, 1693, 2359, 2391, 2439, 2535, 3609, 3705, 3753, 3785, 4367, 4591, 4935, 5031, 5235, 5267, 5691, 5851, 6437, 6597, 7021, 7053, 7257, 7353, 7697, 7921, 8607 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS An XOR-triangle is an inverted 0-1 triangle formed by choosing a top row and having each entry in the subsequent rows be the XOR of the two values above it. If n is in the sequence, then so is A030101(n), the binary reversal of n. All positive terms are odd because each side must begin with (and therefore end with) a 1. The number of terms with a given binary length (A070939) is either 0 or a power of 2. This is because the "all sides are equal" property is equivalent to being the solution to a system of linear equations over the field of integers modulo 2. If x, y, and z are in the sequence and have the same binary length, then x XOR y XOR z is also in the sequence, where XOR is the nim sum (A003987). The second row in triangle has binary expansion given by A038554. The resemblance to 1D CA stems from that it's the same rule as "Rule 90", aka "Pascal's triangle reduced modulo 2" aka Sierpinski Gasket. - Antti Karttunen May 06 2020 LINKS Rémy Sigrist, Table of n, a(n) for n = 1..10000 Michael De Vlieger, Triangles illustrating the initial terms [Keyword "look" refers to this link] "DSM" MathOverflow user, Number triangle Rémy Sigrist, C program for A334556 Rémy Sigrist, PARI program for A334556 EXAMPLE The sequence contains 83 = 1010011_2. Reading clockwise, all sides of the corresponding XOR triangle are 1010011.   1 0 1 0 0 1 1    1 1 1 0 1 0     0 0 1 1 1      0 1 0 0       1 1 0        0 1         1 MATHEMATICA Select[Range[10^4], Block[{n = #, m, w}, m = IntegerLength[n, 2]; (Reverse /@ Transpose[#] /. -1 -> Nothing) == w &@ MapIndexed[PadRight[#, m, -1] &, Set[w, NestList[Map[BitXor @@ # &, Partition[#, 2, 1]] &, IntegerDigits[n, 2], m - 1]]]] &] (* Michael De Vlieger, May 06 2020 *) PROG (C) See Links section. (PARI) is(n) = { my (m=#binary(n)-1, x=n); for (k=0, m, if (bittest(n, m-k)!=bittest(x, 0) || bittest(x, m-k)!=bittest(n, k), return (0)); x=bitxor(x, x\2)); return (1) } \\ Rémy Sigrist, May 07 2020 (PARI) See Links section. CROSSREFS Cf. A003987, A030101, A038554, A070939. Sequence in context: A053649 A104151 A088263 * A230665 A132238 A132241 Adjacent sequences:  A334553 A334554 A334555 * A334557 A334558 A334559 KEYWORD base,nonn,nice,look AUTHOR Peter Kagey, May 06 2020 EXTENSIONS 0 prepended by Rémy Sigrist, May 07 2020 STATUS approved

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Last modified September 18 11:20 EDT 2021. Contains 347518 sequences. (Running on oeis4.)