login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A331040 Numerator of squared radius of inscribed circle of a triangle with integer sides i <= j <= k, such that the number of triangles with this radius sets a new record. Denominators are A331041. 8
1, 35, 3, 7, 3, 15, 8, 35, 55, 63, 95, 119, 135, 56, 231, 255, 80, 351, 455, 495, 855, 216, 224, 1071, 360 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The radius rho of the inscribed circle of a triangle (a,b,c) is rho = sqrt((s-a)*(s-b)*(s-c)/s), with s=(a+b+c)/2. For given integer values of a <= b and a rational target value r2 of the squared incircle radius, c is given by the two positive real roots of the polynomial P(a,b,x,r2) = x^3 - x^2 * (a+b) + x * (4*r2-(b-a)^2) + (a+b)^3 + 4*(a+b)*(r2-a*b). P(a,b,x,r2) = 0 may have 0, 1 or 2 positive integer solutions.

The potential ranges of the side lengths of the triangles can be determined in analogy to the ranges for the case of integer radii of the incircles, see A120062 for the relevant formulas and sequences.

LINKS

Table of n, a(n) for n=1..25.

Hugo Pfoertner, Illustration of side lengths for terms a(13)-a(25).

EXAMPLE

b(1) = a(1)/A331041(1) = 1/12: Triangle (1,1,1) has the least possible radius of incircle = sqrt(1/12).

b(2) = a(2)/A331041(2) = 35/52: Triangles (2,18,19) and (3,4,6) are the first occurrence of more than one triangle with the same radius of their incircles. rho = sqrt(35/52) in both cases.

b(3) = a(3)/A331041(3) = 3/4: Triangles are (2,7,7), (3,3,3), and (3,5,7).

b(4) = a(4)/A331041(4) = 7/4: (3,12,12), (3,22,23), (4,5,6), (5,18,22), (6,11,16) are the A331043(4) = 5 triangles with rho^2 = b(4).

b(15) = 231/4 includes the rare case, where two distinct integer solutions for the same pair of sides a and b exist: (20,37,38) and (20,37,39), both with rho^2=231/4 and thus contributing 2 of the A331043(15)=84 triangles with this squared radius of the incircle.

PROG

(PARI) \\ Only suitable for demonstration of initial terms

rh2(a, b, c)={my(s=(a+b+c)/2); (s-a)*(s-b)*(s-c)/s};

lim_a(x)=ceil(4*(x^2+2));

lim_b(x)=ceil(4*(x^4+2*x^2+1));

target=35/4; v=vector(333938); n=0;

for(a=1, lim_a(sqrt(target)), for(b=a, lim_b(sqrt(target)), for(c=b, a+b-1, f=rh2(a, b, c); v[n++]=f)));

v=vecsort(v); print("A331040 A331041 A331043"); print(numerator(v[1]), " ", denominator(v[1]), " ", 1); m=0; mm=0; for(k=2, #v, if(v[k]>target, break); if(v[k]==v[k-1], m++; if(m>mm&&v[k+1]>v[k], print(numerator(v[k]), " ", denominator(v[k]), " ", m); mm=m), m=1));

CROSSREFS

Cf. A057721, A082044, A120062, A120572, A331012.

Cf. A331041 (corresponding denominators), A331042 (floor(4*a(n)/A331042(n)), A331043 (records of numbers of triangles).

Sequence in context: A059023 A327004 A061045 * A272683 A037934 A329714

Adjacent sequences:  A331037 A331038 A331039 * A331041 A331042 A331043

KEYWORD

nonn,frac,more

AUTHOR

Hugo Pfoertner, Jan 11 2020

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 10 11:32 EDT 2021. Contains 342845 sequences. (Running on oeis4.)