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 A068776 a(0) = 1; for n > 0, a(n) is the smallest triangular number which is a (proper) multiple of a(n-1). 4
 1, 3, 6, 36, 2016, 1493856, 579616128, 11013286048128, 1004811205553955491328, 1897191992473259952000123882626029056, 5012064437680248664058937311304485563631765718940918773832320000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS EXAMPLE a(2) = 6, since 6 = 2*a(1) and 6 is a triangular number. MATHEMATICA pm1[{k_}] := {1, k-1}; pm1[lst_] := Module[{a, m, v}, a=lst[[1]]; m=Times@@Rest[lst]; v=pm1[Rest[lst]]; Union[ChineseRemainder[{1, #}, {a, m}]&/@v, ChineseRemainder[{-1, #}, {a, m}]&/@v]]; nexttri[1]=3; nexttri[n_] := Module[{s}, s=(pm1[Power@@#&/@FactorInteger[4n]]^2-1)/8; For[i=1, True, i++, If[s[[i]]>n, Return[s[[i]]]]]]; a[0]=1; a[n_] := a[n]=nexttri[a[n-1]]; (* First do <= 3, pm1[lst] is the list of numbers less than their product and == 1 or -1 (mod every element of lst). nexttri[n] is the smallest triangular number >n and divisible by n. *) PROG (PARI) {a068776(m)=local(k, q, n); k=1; q=k*(k+1)/2; while(q0, k++; q=q+k))} CROSSREFS Cf. A000217, A068142. Sequence in context: A174666 A211893 A093800 * A025596 A172361 A114038 Adjacent sequences:  A068773 A068774 A068775 * A068777 A068778 A068779 KEYWORD nonn AUTHOR Klaus Brockhaus, Mar 01 2002 EXTENSIONS Edited by Dean Hickerson, Mar 09 2002 STATUS approved

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Last modified January 27 05:41 EST 2021. Contains 340455 sequences. (Running on oeis4.)