A229461 -id:A229461 - cp's OEIS frontend

7, 15, 28, 60, 112, 240

Offset: 1

Suggested by Eike Hertel, Hugo Pfoertner, Sep 24 2013

Idoneal numbers not of the form 7 or 15 times a square. - Hugo Pfoertner, Aug 13 2017

Eike Hertel, Reguläre Dreieckspflasterungen konvexer Polygone, Jenaer Schriften zur Mathematik und Informatik Math/Inf/01/13, 2013 (Preprint).
Eike Hertel, Christian Richter, Tiling Convex Polygons with Congruent Equilateral Triangles, Discrete Comput Geom (2014) 51:753-759.
MathOverflow, Which natural numbers are a square minus a sum of two squares?. Answer by user "Lucia", 2014 Dec 5.

Cf. A000926 (idoneal numbers), A229461 (pentagon exception numbers).

1, 2, 3, 4, 5, 7, 8, 9, 11, 12, 15, 17, 20, 21, 23, 29, 36, 39, 41, 44, 84

Offset: 1

Suggested by Eike Hertel, Hugo Pfoertner, Sep 28 2013

Numbers not expressible as k = 2*(a*c + b*c + b*d) - (a-d)^2 with a,b,c,d >= 1, a < c+d, d < a+b.

See the Hertel paper for the finiteness proof.

Eike Hertel, Reguläre Dreieckspflasterungen konvexer Polygone, Jenaer Schriften zur Mathematik und Informatik, Math/Inf/01/13, 2013 (preprint).
Eike Hertel, Christian Richter, Tiling Convex Polygons with Congruent Equilateral Triangles, Discrete Comput Geom (2014) 51:753-759.
Kival Ngaokrajang, Illustration of initial terms

Cf. A229461 (Pentagon exception numbers).

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