This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A317825 #51 May 22 2025 10:21:48 %S A317825 1,3,0,9,-4,0,7,27,-18,-12,23,0,13,21,-6,81,-64,-54,73,-36,57,69,-46, %T A317825 0,25,39,-12,63,-34,-18,49,243,-210,-192,227,-162,199,219,-180,-108, %U A317825 149,171,-128,207,-162,-138,185,0,49,75,-24,117,-64,-36,91,189,-132,-102,161,-54,115,147,-84,729,-664,-630,697,-576 %N A317825 a(1) = 1, a(n) = 3*a(n/2) if n is even, a(n) = n - a(n-1) if n is odd. %C A317825 Sequence has an elegant fractal-like scatter plot, situated (approximately) symmetrically over X-axis. %C A317825 This sequence can also be generalized with some modifications. Let f_k(1) = 1. f_k(n) = floor(k*a(n/2)) if n is even, f_k(n) = n - f_k(n-1) if n is odd. This sequence is a(n) = f_k(n) where k = 3. For example, if k is e (A001113), then recurrence also provides a curious fractal-like structure that has some similarities with a(n). See Links section for their plots. %C A317825 A scatterplot of (Sum_{i = 1..2*n} a(i)) - n^2 gives a similar plot as for a(n). - _A.H.M. Smeets_, Sep 01 2018 %H A317825 Antti Karttunen, <a href="/A317825/b317825.txt">Table of n, a(n) for n = 1..16383</a> %H A317825 Altug Alkan, <a href="/A317825/a317825.png">A scatterplot of a(n) for n <= 2^15-1</a> %H A317825 Altug Alkan, <a href="/A317825/a317825_1.png">A scatterplot of f_e(n) for n <= 2^15-1</a> %H A317825 Altug Alkan, <a href="/A317825/a317825_2.png">A scatterplot of (A317825(n), abs(A318303(n)))</a> %H A317825 Rémy Sigrist, <a href="/A317825/a317825_3.png">A colored scatterplot of (A317825(n), abs(A318303(n))) for n = 1..2^20-1 </a> (where the color is function of n) %F A317825 From _A.H.M. Smeets_, Sep 01 2018: (Start) %F A317825 Sum_{i = 1..2*n-1} a(i) = n^2 for n >= 0. %F A317825 Sum_{i = 1..2*n} a(i) = 3*a(n) + n^2 for n >= 0, a(0) = 0. %F A317825 Sum_{i = 1..36*2^n} a(i) = 162*A085350(n) for n >= 0. %F A317825 Lim_{n -> infinity} a(n)/n^2 = 0. %F A317825 Lim_{n -> infinity} (Sum_{i = 1..n} a(i))/n^2 = 1/4. (End) %t A317825 Nest[Append[#1, If[EvenQ[#2], 3 #1[[#2/2]], #2 - #1[[-1]] ]] & @@ {#, Length@ # + 1} &, {1}, 67] (* _Michael De Vlieger_, Aug 22 2018 *) %o A317825 (PARI) A317825(n) = if(1==n,n,if(!(n%2),3*A317825(n/2),n-A317825(n-1))); %o A317825 (Python) %o A317825 aa = [0] %o A317825 a,n = 0,0 %o A317825 while n < 16383: %o A317825 n = n+1 %o A317825 if n%2 == 0: %o A317825 a = 3*aa[n//2] %o A317825 else: %o A317825 a = n-a %o A317825 aa = aa+[a] %o A317825 print(n,a) # _A.H.M. Smeets_, Sep 01 2018 %o A317825 (Magma) [n eq 1 select 1 else IsEven(n) select 3*Self(n div 2) else n- Self(n-1): n in [1..80]]; // _Vincenzo Librandi_, Sep 03 2018 %Y A317825 Cf. A318265, A318303. %K A317825 sign,look %O A317825 1,2 %A A317825 _Altug Alkan_ and _Antti Karttunen_, Aug 22 2018