Universal sums of generalized octagonal numbers

Abstract

An integer of the form $P_8(x)=3x^2-2x$ for some integer $x$ is called a generalized octagonal number. A quaternary sum $\Phi_{a,b,c,d}(x,y,z,t)=aP_8(x)+bP_8(y)+cP_8(z)+dP_8(t)$ of generalized octagonal numbers is called {\it universal} if $\Phi_{a,b,c,d}(x,y,z,t)=n$ has an integer solution $x,y,z,t$ for any positive integer $n$. In this article, we show that if $a=1$ and $(b,c,d)=(1,3,3), (1,3,6), (2,3,6), (2,3,7)$ or $(2,3,9)$, then $\Phi_{a,b,c,d}(x,y,z,t)$ is universal. These were conjectured by Sun in \cite {sun}. We also give an effective criterion on the universality of an arbitrary sum $a_1 P_8(x_1)+a_2P_8(x_2)+\cdots+a_kP_8(x_k)$ of generalized octagonal numbers, which is a generalization of "$15$-theorem" of Conway and Schneeberger.