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A236652
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Positive integers n such that n^2 divided by the digital root of n is a square.
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2
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1, 4, 9, 10, 18, 19, 22, 27, 28, 36, 37, 40, 45, 46, 54, 55, 58, 63, 64, 72, 73, 76, 81, 82, 90, 91, 94, 99, 100, 108, 109, 112, 117, 118, 126, 127, 130, 135, 136, 144, 145, 148, 153, 154, 162, 163, 166, 171, 172, 180, 181, 184, 189, 190, 198, 199, 202, 207
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OFFSET
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1,2
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LINKS
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Colin Barker, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,1,-1).
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FORMULA
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a(n) = a(n-1)+a(n-5)-a(n-6).
G.f.: x*(8*x^4+x^3+5*x^2+3*x+1) / ((x-1)^2*(x^4+x^3+x^2+x+1)).
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EXAMPLE
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18 is in the sequence because the digital root of 18 is 9, and 18^2/9 = 36 = 6^2.
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PROG
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(PARI) s=[]; for(n=1, 300, d=(n-1)%9+1; if(n^2%d==0 && issquare(n^2\d), s=concat(s, n))); s
(PARI) Vec(x*(8*x^4+x^3+5*x^2+3*x+1)/((x-1)^2*(x^4+x^3+x^2+x+1)) + O(x^100))
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CROSSREFS
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Cf. A010888, A236653.
Sequence in context: A155566 A305231 A312832 * A236748 A102837 A356417
Adjacent sequences: A236649 A236650 A236651 * A236653 A236654 A236655
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KEYWORD
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nonn,base,easy
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AUTHOR
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Colin Barker, Jan 29 2014
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STATUS
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approved
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