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A292521
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a(n) = a(n-2) - 2a(n-3) + a(n-4) for n>3, with a(0)=2, a(1)=0, a(2)=1, a(3)=-1, a sequence related to Pellian numbers.
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1
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2, 0, 1, -1, 3, -3, 6, -10, 15, -25, 41, -65, 106, -172, 277, -449, 727, -1175, 1902, -3078, 4979, -8057, 13037, -21093, 34130, -55224, 89353, -144577, 233931, -378507, 612438, -990946, 1603383, -2594329, 4197713, -6792041, 10989754, -17781796, 28771549, -46553345
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OFFSET
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0,1
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COMMENTS
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Successive differences begin:
2, 0, 1, -1, 3, -3, 6, -10, 15, -25, ... = a(n)
-2, 1, -2, 4, -6, 9, -16, 25, -40, 66, ... = b(n)
3, -3, 6, -10, 15, -25, 41, -65, 106, -172, ... = a(n+4)
-6, 9, -16, 25, -40, 66, -106, 171, -278, 449, ... = b(n+4)
15, -25, 41, -65, 106, -172, 277, -449, 727, -1175, ... = a(n+8)
...
Main diagonal [2] 1, 6, 25, 106, 449, ... (omitting first term) is A048875 (Pellian numbers with second term 6).
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LINKS
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FORMULA
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G.f.: (2 - x^2 + 3*x^3) / (1 - x^2 + 2*x^3 - x^4).
a(n) = A291660(-n) (negative indices computed using A291660 sequence function).
a(n) = (1/15)*2^(n-1)*((9*sqrt(5)+30)/(1+sqrt(5))^n + (30-9*sqrt(5))/(1- sqrt(5))^n - 5*sqrt(3)*2^(1-n)*sin(n*Pi/3)).
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MATHEMATICA
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LinearRecurrence[{0, 1, -2, 1}, {2, 0, 1, -1}, 40]
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PROG
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(PARI) x='x+O('x^99); Vec((2-x^2+3*x^3)/(1-x^2+2*x^3-x^4)) \\ Altug Alkan, Sep 18 2017
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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