Two right triangles with the two legs of the top one equal to the leg and hypotenuse of the bottom one. For these lengths, a 2 {\displaystyle a^{2}} , b 2 {\displaystyle b^{2}} , and c 2 {\displaystyle c^{2}} form an arithmetic progression separated by a gap of d 2 {\displaystyle d^{2}} . It is not possible for all four lengths a {\displaystyle a} , b {\displaystyle b} , c {\displaystyle c} , and d {\displaystyle d} to be integers.