How to know if this filter is causal?
&quot;The word causal indicates that the filter output depends only on past and present inputs. A filter whose output also depends on future inputs is non-causal, whereas a filter whose output depends only on future inputs is anti-causal&quot;
The input is \$x[n]\$ and output is \$y[n]\$.
And since \$n\$ is present and \$n-1\$ is past values, it's causal.