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%I #25 Jun 04 2021 22:49:45
%S 6,15,25,36,48,61,75,90,106,123,141,160,180,201,223,246,270,295,321,
%T 348,376,405,435,466,498,531,565,600,636,673,711,750,790,831,873,916,
%U 960,1005,1051,1098,1146,1195,1245,1296,1348,1401,1455,1510,1566,1623
%N Truncated triangular numbers: n*(n+1)/2 - 3*t*(t+1)/2 with t=4.
%C Equals binomial transform of [6, 9, 1, 0, 0, 0, ...]. - _Gary W. Adamson_, Aug 25 2009
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F a(n) = A000217(n)-30, n>7.
%F a(n) = n + a(n-1) with n>8, a(8)=6. - _Vincenzo Librandi_, Aug 06 2010
%F G.f.: x^8*(6-3*x-2*x^2)/(1-x)^3. - _Colin Barker_, Mar 19 2012
%t Table[n*(n + 1)/2 - 30, {n, 8, 70}] (* _Stefan Steinerberger_, Mar 31 2006 *)
%t Select[Accumulate[Range[60]]-30,#>0&] (* _Harvey P. Dale_, Apr 13 2011 *)
%o (PARI) a(n)=n*(n+1)/2-30 \\ _Charles R Greathouse IV_, Nov 10 2015
%K nonn,easy,nice
%O 8,1
%A Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999
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