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A091791 Least multiple of n such that the n-th partial concatenation is a multiple of the n-th odd number (i.e., 2n-1). 2

%I

%S 1,2,15,20,25,6,91,80,99,170,88,60,325,350,225,512,646,540,114,60,672,

%T 704,460,696,675,78,135,560,493,2010,589,992,2145,1768,35,900,2072,

%U 1900,3159,2680

%N Least multiple of n such that the n-th partial concatenation is a multiple of the n-th odd number (i.e., 2n-1).

%e 3 divides 12, 5 divides 1215, 7 divides 121520 etc.

%p A055642 := proc(n) ilog10(n)+ 1; end: Lcat := proc(L) local resul,i ; resul := op(1,L) ; for i from 2 to nops(L) do resul := resul*10^A055642(op(i,L))+op(i,L) ; od ; RETURN(resul) ; end: A091791 := proc(nmax) local a,k,n ; a := [1] ; for n from 2 to nmax do k :=1 ; while Lcat([op(a),k*n]) mod (2*n-1) <> 0 do k := k+1 ; od ; a := [op(a),k*n] ; od; a ; end: A091791(40); # _R. J. Mathar_, Jul 20 2007

%Y Cf. A091792.

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Feb 18 2004

%E More terms from _R. J. Mathar_, Jul 20 2007

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Last modified July 12 10:09 EDT 2020. Contains 335657 sequences. (Running on oeis4.)