René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Inconsistent evaluation of tf/ss systems with pole at the origin","articleBody":"Systems that have a pole at the origin return inconsistent values:\r\n```\r\nimport control as ct\r\nsys_tf = ct.tf([1], [1, 0])\r\nsys_ss = ct.tf2ss(sys_tf)\r\n\r\nsys_tf(0)        # returns (inf+0j) with divide by zero warning\r\nsys_tf(0j)       # returns (inf+nanj) with divide by zero warning\r\nsys_ss(0)        # LinAlgError: Singular matrix\r\nsys_tf.dcgain()  # returns array(inf)\r\nsys_ss.dcgain()  # returns array(nan)\r\n```\r\nI think all of these should return `inf` (or `array(inf)`).\r\n\r\nIf you have a pole and zero at the origin, you get `nan` or 1:\r\n```\r\nsys_tf = ct.tf([1, 0], [1, 0])\r\nsys_ss = ct.tf2ss(sys_tf)\r\n\r\nsys_tf(0)        # returns (nan+0j) with invalid value warning\r\nsys_ss(0)        # returns (1+0j) [state space system is empty]\r\nsys_tf.dcgain()  # returns array(nan)\r\nsys_ss.dcgain()  # returns (1+0j) [state space system is empty]\r\n```\r\nI think all of these cases should produce `nan`.\r\n\r\nI have a partial fix in a branch, but want to see what people think the \"right\" answer is before committing it.\r\n\r\n","author":{"url":"https://github.com/murrayrm","@type":"Person","name":"murrayrm"},"datePublished":"2021-02-06T07:11:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/532/python-control/issues/532"}