I’m following Asmar’s book on PDE and used gnuplot to answer question §2.1 2c: find period for cos(mx) & sketch the graph. This was my command for:

set xrange [0:14]

set yrange [-5:5]

b = 2*pi

c = 4*pi

set arrow from b, graph 0 to b, graph 1 nohead

set arrow from c, graph 0 to c, graph 1 nohead

plot cos(x),cos(2x),cos(3x),cos(4*x),0