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A061782
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a(n) = smallest positive number m such that m*n is a triangular number.
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5
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1, 3, 1, 7, 2, 1, 3, 15, 4, 1, 5, 3, 6, 2, 1, 31, 8, 2, 9, 6, 1, 3, 11, 5, 12, 3, 13, 1, 14, 4, 15, 63, 2, 4, 3, 1, 18, 5, 2, 3, 20, 5, 21, 12, 1, 6, 23, 11, 24, 6, 3, 15, 26, 7, 1, 21, 3, 7, 29, 2, 30, 8, 6, 127, 5, 1, 33, 2, 4, 3, 35, 28, 36, 9, 4, 21, 3, 1, 39, 26, 40, 10, 41, 14, 7, 11, 5
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OFFSET
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1,2
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LINKS
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FORMULA
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For p an odd prime, a(p) = (p-1)/2. For nonnegative k, a(2^k) = 2^(k+1)-1.
Formula corrected by Nick Singer, Jun 26 2006
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EXAMPLE
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a(4) = 7 as 4*7 = 28 is a triangular number and 7 is the smallest such number.
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MAPLE
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isA000217 := proc(n)
issqr(1+8*n) ;
end proc:
local a;
for a from 1 do
if isA000217(n*a) then
return a;
end if;
end do:
end proc:
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MATHEMATICA
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snt[n_]:=Module[{k=1}, While[!OddQ[Sqrt[1+8k n]], k++]; k]; Array[snt, 100] (* Harvey P. Dale, Feb 15 2017 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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