cp's OEIS Frontend

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.

A195284 Decimal expansion of shortest length of segment from side AB through incenter to side AC in right triangle ABC with sidelengths (a,b,c)=(3,4,5); i.e., decimal expansion of 2*sqrt(10)/3.

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%I A195284 #43 Jan 29 2025 14:58:35
%S A195284 2,1,0,8,1,8,5,1,0,6,7,7,8,9,1,9,5,5,4,6,6,5,9,2,9,0,2,9,6,2,1,8,1,2,
%T A195284 3,5,5,8,1,3,0,3,6,7,5,9,5,5,0,1,4,4,5,5,1,2,3,8,3,3,6,5,6,8,5,2,8,3,
%U A195284 9,6,2,9,2,4,2,6,1,5,8,8,1,4,2,2,9,4,9,8,7,3,8,9,1,9,5,3,3,5,3,0
%N A195284 Decimal expansion of shortest length of segment from side AB through incenter to side AC in right triangle ABC with sidelengths (a,b,c)=(3,4,5); i.e., decimal expansion of 2*sqrt(10)/3.
%C A195284 Apart from the first digit, the same as A176219 (decimal expansion of 2+2*sqrt(10)/3).
%C A195284 The Philo line of a point P inside an angle T is the shortest segment that crosses T and passes through P. Philo lines are not generally Euclidean-constructible.
%C A195284 ...
%C A195284 Suppose that P lies inside a triangle ABC. Let (A) denote the shortest length of segment from AB through P to AC, and likewise for (B) and (C). The Philo number for ABC and P is here introduced as the normalized sum ((A)+(B)+(C))/(a+b+c), denoted by Philo(ABC,P).
%C A195284 ...
%C A195284 Listed below are examples for which P=incenter (the center, I, of the circle inscribed in ABC, the intersection of the angle bisectors of ABC); in this list, r'x means sqrt(x), and t=(1+sqrt(5))/2 (the golden ratio).
%C A195284 a....b....c.......(A).......(B).......(C)....Philo(ABC,I)
%C A195284 3....4....5.....A195284...A002163...A010466...A195285
%C A195284 5....12...13....A195286...A195288...A010487...A195289
%C A195284 7....24...25....A195290...A010524...15/2......A195292
%C A195284 8....15...17....A195293...A195296...A010524...A195297
%C A195284 28...45...53....A195298...A195299...A010466...A195300
%C A195284 1....1....r'2...A195301...A195301...A163960...A195303
%C A195284 1....2....r'5...A195340...A195341...A195342...A195343
%C A195284 1....3....r'10..A195344...A195345...A195346...A195347
%C A195284 2....3....r'13..A195355...A195356...A195357...A195358
%C A195284 2....5....r'29..A195359...A195360...A195361...A195362
%C A195284 r'2..r'3..r'5...A195365...A195366...A195367...A195368
%C A195284 1....r'2..r'3...A195369...A195370...A195371...A195372
%C A195284 1....r'3..2.....A195348...A093821...A120683...A195380
%C A195284 2....r'5..3.....A195381...A195383...A195384...A195385
%C A195284 r'2..r'5..r'7...A195386...A195387...A195388...A195389
%C A195284 r'3..r'5..r'8...A195395...A195396...A195397...A195398
%C A195284 r'7..3....4.....A195399...A195400...A195401...A195402
%C A195284 1....r't..t.....A195403...A195404...A195405...A195406
%C A195284 t-1..t....r'3...A195407...A195408...A195409...A195410
%C A195284 ...
%C A195284 In the special case that P is the incenter, I, each Philo line, being perpendicular to an angle bisector, is constructible, and (A),(B),(C) can be evaluated exactly.
%C A195284 For the 3,4,5 right triangle, (A)=(2/3)*sqrt(10), (B)=sqrt(5), (C)=sqrt(8), so that Philo(ABC,I)=((2/3)sqrt(10)+sqrt(5)+sqrt(8))/12, approximately 0.59772335.
%C A195284 ...
%C A195284 More generally, for arbitrary right triangle (a,b,c) with a<=b<c, let f=2*a*b/(a+b+c). Then, for P=I,
%C A195284 (A)=f*sqrt(a^2+(b+c)^2)/(b+c),
%C A195284 (B)=f*sqrt(b^2+(c+a)^2)/(c+a),
%C A195284 (C)=f*sqrt(2).
%C A195284 It appears that I is the only triangle center P for which simple formulas for (A), (B), (C) are available. For P=centroid, see A195304.
%D A195284 David Gale, Tracking the Automatic Ant and Other Mathematical Explorations, A Collection of Mathematical Entertainments Columns from The Mathematical Intelligencer, Springer, 1998; see chapter 16.
%D A195284 Clark Kimberling, Geometry In Action, Key College Publishing, 2003, pages 115-116.
%H A195284 Michael Cavers, <a href="http://spikedmath.com/524.html">Spiked Math #524</a> (2012)
%H A195284 Clark Kimberling, <a href="/A195284/a195284.pdf">Geometry In Action</a>, 2003, scanned copy (with permission). See pages 115-116.
%H A195284 <a href="/index/Al#algebraic_02">Index entries for algebraic numbers, degree 2</a>.
%F A195284 Equals (2/3)*sqrt(10).
%e A195284 2.10818510677891955466592902962...
%p A195284 philo := proc(a,b,c) local f, A, B, C, P:
%p A195284 f:=2*a*b/(a+b+c):
%p A195284 A:=f*sqrt((a^2+(b+c)^2))/(b+c):
%p A195284 B:=f*sqrt((b^2+(c+a)^2))/(c+a):
%p A195284 C:=f*sqrt(2):
%p A195284 P:=(A+B+C)/(a+b+c):
%p A195284 print(simplify([A,B,C,P])):
%p A195284 print(evalf([A,B,C,P])): end:
%p A195284 philo(3,4,5); # _Georg Fischer_, Jul 18 2021
%t A195284 a = 3; b = 4; c = 5;
%t A195284 h = a (a + c)/(a + b + c); k = a*b/(a + b + c); (* incenter *)
%t A195284 f[t_] := (t - a)^2 + ((t - a)^2) ((a*k - b*t)/(a*h - a*t))^2;
%t A195284 s = NSolve[D[f[t], t] == 0, t, 150]
%t A195284 f1 = (f[t])^(1/2) /. Part[s, 4]
%t A195284 RealDigits[%, 10, 100] (* (A) 195284 *)
%t A195284 f[t_] := (b*t/a)^2 + ((b*t/a)^2) ((a*h - a*t)/(b*t - a*k))^2
%t A195284 s = NSolve[D[f[t], t] == 0, t, 150]
%t A195284 f2 = (f[t])^(1/2) /. Part[s, 1]
%t A195284 RealDigits[%, 10, 100] (* (B) A002163 *)
%t A195284 f[t_] := (t - a)^2 + ((t - a)^2) (k/(h - t))^2
%t A195284 s = NSolve[D[f[t], t] == 0, t, 150]
%t A195284 f3 = (f[t])^(1/2) /. Part[s, 4]
%t A195284 RealDigits[%, 10, 100] (* (C) A010466 *)
%t A195284 (f1 + f2 + f3)/(a + b + c)
%t A195284 RealDigits[%, 10, 100] (* Philo(ABC,I) A195285 *)
%o A195284 (PARI) (2/3)*sqrt(10) \\ _Michel Marcus_, Dec 24 2017
%Y A195284 Cf. A002163, A010466, A195285, A195304.
%K A195284 nonn,cons
%O A195284 1,1
%A A195284 _Clark Kimberling_, Sep 14 2011
%E A195284 Table and formulas corrected by _Georg Fischer_, Jul 17 2021