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7, 13, 17, 31, 37, 59, 61, 65, 77, 83, 89, 97, 107, 127, 129, 131, 133, 145, 153, 165, 169, 179, 195, 197, 201, 221, 227, 235, 245, 249, 261, 269, 281, 293, 297, 303, 321, 325, 345, 369, 373, 381, 393, 399, 405, 409, 417, 421, 425, 427, 442, 443, 447, 455, 465
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graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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COMMENTS
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LINKS
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MATHEMATICA
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Block[{s, nn}, nn = 50; s = Map[FactorInteger[#][[-1, 1]] &, 2^Range[4 nn] - 1]; Select[Range[nn], s[[#]] == s[[2 #]] == s[[4 #]] &]] (* Michael De Vlieger, Dec 15 2022 *)
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PROG
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(PARI) f(n) = vecmax(factor(2^n-1)[, 1]); \\ A005420
isok(n) = my(x=f(2*n)); (f(n) == x) && (f(4*n) == x);
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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