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Search: id:A158446
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| 20, 95, 220, 395, 620, 895, 1220, 1595, 2020, 2495, 3020, 3595, 4220, 4895, 5620, 6395, 7220, 8095, 9020, 9995, 11020, 12095, 13220, 14395, 15620, 16895, 18220, 19595, 21020, 22495, 24020, 25595, 27220, 28895, 30620, 32395, 34220, 36095
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OFFSET
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1,1
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COMMENT
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If A=[A158446] 25*n.^2-5 (n>0, 20, 95, 220,.,); Y=[A005843] 2*n (n>0, 2, 4, 6,.,); X = [A158447] 10*n^2-1 (n>0, 9, 39, 89, .,), we have, for all terms, Pell's equation X^2-A*Y^2=1. Example: 9^2-20*2^2=1; 39^2-95*4^2=1; 89^2-220*6^2=1.
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LINKS
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Edward Everett Withford, Pell Equation
Vincenzo Librandi, X^2-AY^2=1
Wolfram MathWorld, Pell Equation
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FORMULA
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a(n)=25*n^2-5 (n>0)
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EXAMPLE
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For n=1, a(1)=20; n=2, a(2)=95; n=3, a(3)=220
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CROSSREFS
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Cf. A005843, A158447
Sequence in context: A144359 A124948 A126407 this_sequence A039610 A157429 A128676
Adjacent sequences: A158443 A158444 A158445 this_sequence A158447 A158448 A158449
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KEYWORD
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nonn
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AUTHOR
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Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Mar 19 2009
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