Yn+1-2Yn+Yn-1=0, Yn+1=2Yn-Yn-1.
Let Y0=a and Y1=b, so that the whole series is based on these two values.
n=1: Y2=2b-a,
n=2: Y3=2Y2-b=4b-2a-b=3b-2a,
n=3: Y4=2Y3-Y2=6b-4a-2b+a=4b-3a,
n=4: Y5=2Y4-Y3=8b-6a-3b+2a=5b-4a.
The pattern is Yn=nb-a(n-1) or n(b-a)+a. When a=b, Yn=a, so all terms are a.
If b=a+p, Yn=an+pn-an+a=pn+a, so the series is a, a+p, a+2p, a+3p, ..., which is an arithmetic progression with first term=a and common difference is p.