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A061782
a(n) = smallest positive number m such that m*n is a triangular number.
5
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
OFFSET
1,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..20000 (first 1000 terms from Harvey P. Dale)
FORMULA
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
EXAMPLE
a(4) = 7 as 4*7 = 28 is a triangular number and 7 is the smallest such number.
MAPLE
isA000217 := proc(n)
issqr(1+8*n) ;
end proc:
A061782 := proc(n)
local a;
for a from 1 do
if isA000217(n*a) then
return a;
end if;
end do:
end proc:
seq(A061782(n), n=1..40) ; # R. J. Mathar, Oct 03 2014
MATHEMATICA
snt[n_]:=Module[{k=1}, While[!OddQ[Sqrt[1+8k n]], k++]; k]; Array[snt, 100] (* Harvey P. Dale, Feb 15 2017 *)
CROSSREFS
Sequence in context: A146265 A146217 A146007 * A074625 A284877 A169659
KEYWORD
nonn,easy
AUTHOR
Amarnath Murthy, May 24 2001
EXTENSIONS
Corrected and extended by Matthew Conroy, May 28 2001
STATUS
approved