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A178915 Rearrangement of natural numbers so that every partial sum is composite. 1

%I #14 Jun 30 2023 15:39:36

%S 4,2,3,1,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,

%T 27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,

%U 50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72

%N Rearrangement of natural numbers so that every partial sum is composite.

%C a(n) = n for n > 4.

%C Except for the integers 1 & 4 which are interchanged, the sequence is in order. Proof: Except for the first three triangular numbers (A000217), {0, 1, 3}, they are all composite. - _Robert G. Wilson v_, Jun 27 2010

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2, -1).

%F G.f.: 3 - 3*x^3 + 1/(x-1)^2. - _Sergei N. Gladkovskii_, Oct 16 2012

%e Partial sums are 4,6,9,10,15,21,...

%t f[s_List] := Block[{k = 0, t = Plus @@ s}, While[MemberQ[s, k] || PrimeQ[t + k] || t + k < 2, k++ ]; Append[s, k]]; Rest@ Nest[f, {0}, 72] (* _Robert G. Wilson v_, Jun 27 2010 *)

%K easy,nonn

%O 1,1

%A _Amarnath Murthy_, Jun 23 2010

%E a(40)-a(72) from _Robert G. Wilson v_, Jun 27 2010

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