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A056527
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Numbers where iterated sum of digits of square settles down to a cyclic pattern (in fact 13, 16, 13, 16, ...).
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4
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2, 4, 5, 7, 11, 13, 14, 16, 20, 22, 23, 25, 29, 31, 32, 34, 38, 40, 41, 43, 47, 49, 50, 52, 56, 58, 59, 61, 65, 67, 68, 70, 74, 76, 77, 79, 83, 85, 86, 88, 92, 94, 95, 97, 101, 103, 104, 106, 110, 112, 113, 115, 119, 121, 122, 124, 128, 130, 131, 133, 137, 139, 140
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OFFSET
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1,1
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COMMENTS
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Numbers == 2, 4, 5 or 7 mod 9, i.e. such that n^4 is not congruent to n^2 mod 9.
Numbers congruent to {2, 4, 5, 7} mod 9.
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LINKS
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FORMULA
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a(n) = a(n-1) + a(n-4) - a(n-5) for n>5. - Harvey P. Dale, Apr 05 2015
G.f.: x*(2 + 2*x + x^2 + 2*x^3 + 2*x^4) / ((1 - x)^2*(1 + x)*(1 + x^2)).
a(n) = (-9 + (-1)^(1+n) - (3-3*i)*(-i)^n - (3+3*i)*i^n + 18*n) / 8 where i=sqrt(-1).
(End)
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EXAMPLE
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a(1)=2 because iteration starts 2, 4, 7, 13, 16, 13, 16, ....
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MATHEMATICA
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Flatten[Table[9n+{2, 4, 5, 7}, {n, 0, 20}]] (* or *) LinearRecurrence[{1, 0, 0, 1, -1}, {2, 4, 5, 7, 11}, 100] (* Harvey P. Dale, Apr 05 2015 *)
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PROG
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(PARI) Vec(x*(2 + 2*x + x^2 + 2*x^3 + 2*x^4) / ((1 - x)^2*(1 + x)*(1 + x^2)) + O(x^80)) \\ Colin Barker, Dec 19 2017
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CROSSREFS
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Cf. A004159 for sum of digits of square, A056020 where iteration settles to 1, A056020 where iteration settles to 9, also A056528, A056529. Unhappy numbers A031177 deal with iteration of square of sum of digits not settling to a single result.
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KEYWORD
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base,easy,nonn
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AUTHOR
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STATUS
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approved
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