The stable homotopy groups of spheres are the direct sum of the image of the -homomorphism, and the kernel of the Adams -invariant, a homomorphism from these groups to . Roughly speaking, the image of the -homomorphism is the subgroup of "well understood" or "easy" elements of the stable homotopy groups. These well understood elements account for most elements of the stable homotopy groups of spheres in small dimensions. The quotient of by the image of the -homomorphism is considered to be the "hard" part of the stable homotopy groups of spheres . (Adams also introduced certain order 2 elements of for , and these are also considered to be "well understood".) Tables of homotopy groups of spheres sometimes omit the "easy" part to save space.