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A175117 Numbers k such that Sum_(i=1..k) prime(i)*(-1)^(i+1) is a square. 1
3, 9, 55, 181, 215, 459, 1361, 1417, 1623, 2047, 2445, 4685, 6687, 7353, 7785, 7925, 8489, 10333, 10515, 11179, 14087, 15227, 15829 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Numbers k such that (-1)^k*A008347(k) is a perfect square. - R. J. Mathar, Feb 21 2010
LINKS
EXAMPLE
2 - 3 + 5 = 4, thus a(1)=3;
2 - 3 + 5 - 7 + 11 - 13 + 17 - 19 + 23 = 16, thus a(2)=9.
MATHEMATICA
Flatten[Position[Accumulate[Times@@@Partition[Riffle[Prime[Range[ 20000]], {1, -1}, {2, -1, 2}], 2]], _?(IntegerQ[Sqrt[#]]&)]] (* Harvey P. Dale, Mar 19 2013 *)
CROSSREFS
Cf. A000040.
Sequence in context: A001194 A032179 A233189 * A245117 A183303 A075979
KEYWORD
easy,nonn
AUTHOR
Ctibor O. Zizka, Feb 14 2010
EXTENSIONS
Six more terms from R. J. Mathar, Feb 21 2010
STATUS
approved

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Last modified April 22 08:40 EDT 2024. Contains 371893 sequences. (Running on oeis4.)