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Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!) A280296 Squarefree numbers k such that 2^k - 1 is divisible by a square > 1. 5 6, 21, 30, 42, 66, 78, 102, 105, 110, 114, 138, 155, 174, 186, 210, 222, 231, 246, 253, 258, 273, 282, 310, 318, 330, 354, 357, 366, 390, 399, 402, 426, 438, 462, 465, 474, 483, 498, 506, 510, 534, 546, 570, 582, 602, 606, 609, 618, 642, 651, 654, 678, 690, 714, 759, 762, 770, 777, 786, 798, 822 (list; graph; refs; listen; history; text; internal format) OFFSET 1,1 COMMENTS Intersection of A049094 and A005117. - Michel Marcus, Dec 31 2016 LINKS Amiram Eldar, Table of n, a(n) for n = 1..89 EXAMPLE 6 is in this sequence because 2^6 - 1 = 63 is divisible by 9 = 3^2. PROG (Magma) [n: n in [1..200] | IsSquarefree(n) and not IsSquarefree(2^n-1)]; CROSSREFS Cf. A005117, A049094, A237043, A280203, A280208, A280209. Sequence in context: A034897 A347875 A287165 * A325407 A239920 A173622 Adjacent sequences: A280293 A280294 A280295 * A280297 A280298 A280299 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, Dec 31 2016 EXTENSIONS a(38)=498 inserted by Amiram Eldar, Oct 23 2019 STATUS approved

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Last modified July 19 06:32 EDT 2024. Contains 374389 sequences. (Running on oeis4.)