A014498 and JHC's extraversion
Antreas P. Hatzipolakis
xpolakis at otenet.gr
Mon Jun 5 21:40:17 CEST 2000
ID Number: A014498
Sequence: 1,2,3,3,6,5,4,10,5,12,7,15,14,6,15,20,9,21,7,18,28,11,8,21,
30,35,22,9,36,24,35,13,42,33,45,10,26,40,44,15,39,11,30,45,
55,56,30,63,52,12,33,66,17,63,65,72
Name: Varying radii of inscribed circles within primitive Pythagorean
triples as a function of increasing values of hypotenuse.
___________________________________________________________________________
The sequence above(*) is inradius-related.
(*) Note for the Hyacinthians: It is from Sloane's _Encycl.of Int. Sequences_
http://www.research.att.com/~njas/sequences/index.html
Let's see if JHC's "extraversion"(**) applies here for the three exradii .
(**): Note for the seqfans: If you want to learn about this and other things
on triangle geometry, join our group Hyacinthos !
Let (z = m^2 + n^2: hypot., (y = 2mn, x = m^2 - n^2): legs) be a pythagorean
triangle.
area(ABC) xy 2mn(m^2-n^2)
r [=inradius] = -------------- = ---- = ------------------------------ =
semiperimeter 2s m^2 + n^2 + 2mn + m^2 - n^2
= n(m-n)
area(ABC) xy 2mn(m^2-n^2)
r_z [= z-exradius] = ---------- = ----- = ------------------------------ =
s-z 2(s-z) -(m^2 + n^2) + 2mn + m^2 - n^2
= m(m+n)
area(ABC) xy 2mn(m^2-n^2)
r_y [= y-exradius] = ---------- = ----- = ------------------------------ =
s-y 2(s-y) m^2 + n^2 - 2mn + m^2 - n^2
= n(m+n)
area(ABC) xy 2mn(m^2-n^2)
r_x [= x-exradius] = ---------- = ----- = ------------------------------- =
s-x 2(s-x) m^2 + n^2 + 2mn - (m^2 - n^2)
= m(m-n)
So, in a pythagorean triangle all 4 in/exradii are integral.
Let's now denote: c = z (hypotenuse)
b = max(y,x) (middle side)
a = min(y,x) (smallest side)
and r, r_c, r_b, r_a the corresponding in/exradii.
TABLE
A046087
A020882 | A046086 A014498
m n c b a r r_c r_b r_a
-------------------------------------------------------------------------
2 1 5 4 3 1 6 3 2
3 2 13 12 5 2 15 10 3
4 1 17 15 8 3 20 12 5
4 3 25 24 7 3 28 21 4
5 2 29 21 20 6 35 15 14
6 1 37 35 12 5 42 30 7
5 4 41 40 9 4 45 36 5
7 2 53 45 28 10 63 35 18
6 5 61 60 11 5 66 55 5
7 4 65 56 33 12 77 44 21
8 1 65 63 16 7 72 56 9
8 3 73 55 48 15 88 40 33
9 2 85 77 36 14 99 22 14
7 6 85 84 13 6 91 78 7
8 5 89 80 39 15 104 65 24
9 4 97 72 65 20 117 52 45
..........
Sequences for the three exradii:
1. r_c sequence: 6,15,20,28,35,42,45,63,66,77,72,88,99,91,104,117,....
Name: Varying radii of c-exscribed circles within primitive Pythagorean
triples as a function of increasing values of hypotenuse c.
Formula: Arrange all primitive Pythagorean triples a,b,c by value of
hypotenuse c; for nth value of c, sequence gives radius of
c-exscribed circle, (a+b+c)/2.
a(n) = (A046086(n) + A046087(n) + A020882(n))/2
Note: This sequence ordered gives:
ID Number: A020886
Sequence: 6,15,20,28,35,42,45,63,66,72,77,88,91,99,104,110,117,120,130,143,
153,156,165,170,187,190,195,204,209,210,221,228,231,238,247,255,266,272,273,276
Name: Ordered semiperimeters of primitive Pythagorean triangles.
2. r_b sequence: 3,10,12,21,15,30,36,35,55,44,56,40,22,78,65,52,.....
Name: Varying radii of b-exscribed circles within primitive Pythagorean
triples as a function of increasing values of hypotenuse c, and
b as middle side.
3. r_c sequence: 2,3,5,4,14,7,5,18,5,21,9,33,14,7,24,45,.....
Name: Varying radii of a-exscribed circles within primitive Pythagorean
triples as a function of increasing values of hypotenuse c, and
a as smallest side.
.
Antreas
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