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A159754
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Numbers n with property that n^2 ends with 81.
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3
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9, 41, 59, 91, 109, 141, 159, 191, 209, 241, 259, 291, 309, 341, 359, 391, 409, 441, 459, 491, 509, 541, 559, 591, 609, 641, 659, 691, 709, 741, 759, 791, 809, 841, 859, 891, 909, 941, 959, 991, 1009, 1041, 1059, 1091, 1109, 1141, 1159, 1191, 1209, 1241, 1259
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OFFSET
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1,1
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COMMENTS
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First 9 terms = 9th row in A111445.
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LINKS
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FORMULA
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a(1) = 9, a(2) = 41, a(n) = a(n-2)+50, n=3,4,...
a(n) = (1/2)*(50*n-25+7*(-1)^n), n=1,2,...
a(n) = a(n-1)+a(n-2)-a(n-3). G.f.: x*(9*x^2+32*x+9) / ((x-1)^2*(x+1)). - Colin Barker, Jul 04 2014
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MAPLE
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a:= n-> iquo(n-1, 2, 'r')*50 +[9, 41][r+1]:
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MATHEMATICA
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Select[Range[1300], PowerMod[#, 2, 100]==81&] (* or *) LinearRecurrence[{1, 1, -1}, {9, 41, 59}, 60] (* Harvey P. Dale, May 17 2018 *)
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PROG
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(PARI) select(n->n^2%100==81, vector(10^3, n, n)) \\ Colin Barker, Jul 04 2014
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CROSSREFS
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A159254 numbers n with property that n^2 ends with 49. A111445 n-th row contains first n numbers whose squares end in n^2.
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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