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URL: https://oeis.org/A161372

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A161372
In Ulam's spiral starting at 101, take the elements not used so far from the two spokes SW, NE, SW, NE, SW, NE ...
0
107, 101, 121, 103, 143, 113, 173, 131, 211, 157, 257, 191, 311, 233, 373, 283, 443, 341, 521, 407, 607, 481, 701, 563, 803, 653, 913, 751, 1031, 857, 1157, 971, 1291, 1093, 1433, 1223, 1583, 1361, 1741, 1507, 1907, 1661, 2081, 1823, 2263, 1993, 2453
OFFSET
1,1
COMMENTS
NE to SW Diagonal of Ulam's Spiral, with 101 at center.
The sequence did not match the original definition. A working definition might be a(2n)=4n^2-10n+107, a(2n-1)=4n^2+2n+101, a(n)=n^2-n+105+2*(-1)^n*(1-2*n), but this seems to be unrelated to Ulam spirals. [R. J. Mathar, Jun 11 2009]
EXAMPLE
SW-NE diagonal in:
137 136 135 134 133 132 131
138 117 116 115 114 113 130
139 118 105 104 103 112 129
140 119 106 101 102 111 128
141 120 107 108 109 110 127
142 121 122 123 124 125 126
143 144 145 146 147 148 149
MATHEMATICA
CoefficientList[Series[(107*x^4-6*x^3-194*x^2-6*x+107)/((1+x)^2*(1-x)^3), {x, 0, 32}], x] (* Georg Fischer, Dec 03 2024 *)
CROSSREFS
Cf. A054569 (SW spoke), A054554 (NE spoke).
Sequence in context: A178546 A096712 A161176 * A099652 A344803 A294744
KEYWORD
nonn,easy
AUTHOR
Milton L. Brown (miltbrown(AT)earthlink.net), Jun 08 2009
EXTENSIONS
Edited by Georg Fischer, Dec 03 2024
STATUS
approved