add more help text

cmd/divisor.go

@@ -59,8 +59,11 @@ func divisorSum(cmd *cobra.Command, args []string) {
 var divisorCmd = &cobra.Command{
 	Use:   "divisor N [N ...]",
 	Short: "Compute the divisor summatory function",
-	Long:  `Compute the divisor summatory function.`,
-	Run:   divisorSum,
+	Long: `Compute the divisor summatory function.
+
+For each argument n, compute D(n) = d(1) + d(2) + d(3) + ... + d(n), where d(n) is the number of divisors of n.`,
+	Args: cobra.MinimumNArgs(1),
+	Run:  divisorSum,
 }
 
 func init() {