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 A117206 Squarefree positive integers k such that 2k-1 is also squarefree. 2
 1, 2, 3, 6, 7, 10, 11, 15, 17, 19, 21, 22, 26, 29, 30, 31, 33, 34, 35, 37, 39, 42, 43, 46, 47, 51, 53, 55, 57, 58, 62, 65, 66, 67, 69, 70, 71, 73, 78, 79, 82, 83, 87, 89, 91, 93, 94, 97, 101, 102, 103, 105, 106, 107, 109, 110, 111, 114, 115, 118, 119, 127, 129, 130, 133 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 2*a(n) - 1 = A117205(n). LINKS EXAMPLE 10 and 2*10 - 1 = 19 are both squarefree, so 10 is in the sequence. MAPLE with(numtheory): a:=proc(n) if abs(mobius(n))>0 and abs(mobius(2*n-1))>0 then n else fi end: seq(a(n), n=1..160); # Emeric Deutsch, Mar 08 2006 MATHEMATICA Select[Range[200], And@@SquareFreeQ/@{#, 2#-1}&] (* Harvey P. Dale, Dec 17 2012 *) CROSSREFS Cf. A005117, A117204, A117205. Sequence in context: A226115 A073170 A014689 * A026443 A204323 A278965 Adjacent sequences:  A117203 A117204 A117205 * A117207 A117208 A117209 KEYWORD nonn AUTHOR Leroy Quet, Mar 02 2006 EXTENSIONS More terms from Emeric Deutsch, Mar 08 2006 STATUS approved

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Last modified May 26 09:42 EDT 2019. Contains 323579 sequences. (Running on oeis4.)