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 A057944 Largest triangular number less than or equal to n; write m-th triangular number m+1 times. 15
 0, 1, 1, 3, 3, 3, 6, 6, 6, 6, 10, 10, 10, 10, 10, 15, 15, 15, 15, 15, 15, 21, 21, 21, 21, 21, 21, 21, 28, 28, 28, 28, 28, 28, 28, 28, 36, 36, 36, 36, 36, 36, 36, 36, 36, 45, 45, 45, 45, 45, 45, 45, 45, 45, 45, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 66, 66, 66, 66, 66, 66 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 LINKS Reinhard Zumkeller, Rows n=0..100 of triangle, flattened FORMULA a(n) = floor((sqrt(1+8*n)-1)/2)*floor((sqrt(1+8*n)+1)/2)/2 = (trinv(n)*(trinv(n)-1))/2 = A000217(A003056(n)) = n - A002262(n) EXAMPLE a(35) = 28 since 28 and 36 are successive triangular numbers and 28 <= 35 < 36. MAPLE A057944 := proc(n)         k := (-1+sqrt(1+8*n))/2 ;         k := floor(k) ;         k*(k+1)/2 ; end proc; # R. J. Mathar, Nov 05 2011 MATHEMATICA f[n_] := Block[{a = Floor@ Sqrt[1 + 8 n]}, Floor[(a - 1)/2]*Floor[(a + 1)/2]/2]; Array[f, 72, 0] t0=0; t1=1; k=1; Table[If[n < t1, t0, k++; t0=t1; t1=t1+k; t0], {n, 0, 72}] PROG (Haskell) a057944 n = a057944_list !! n          -- common flat access a057944_list = concat a057944_tabl a057944' n k = a057944_tabl !! n !! k  -- access when seen as a triangle a057944_row n = a057944_tabl !! n a057944_tabl = zipWith (\$) (map replicate [1..]) a000217_list -- Reinhard Zumkeller, Feb 03 2012 (PARI) a(n)=my(t=(sqrtint(8*n+7)-1)\2); t*(t+1)/2 \\ Charles R Greathouse IV, Jan 26 2013 CROSSREFS Cf. A000217, A003056, A003056, A056944, A057945, A127739. Sequence in context: A108581 A073080 A171601 * A281258 A080607 A013322 Adjacent sequences:  A057941 A057942 A057943 * A057945 A057946 A057947 KEYWORD easy,nonn,tabl AUTHOR Henry Bottomley, Oct 05 2000 EXTENSIONS Keyword tabl added by Reinhard Zumkeller, Feb 03 2012 STATUS approved

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Last modified October 22 19:53 EDT 2019. Contains 328319 sequences. (Running on oeis4.)