December 5, 1985

The investment was huge and the risks enormous as the operators of cable television systems built their businesses over the last 10 years. After laying out hundreds of millions of dollars to wire American homes, they were still at the mercy of fickle consumers and regulatory constraints. Lately, though, there has been growing evidence that the cable system companies were on the right track. The Westinghouse Electric Company, parent of Group W Cable Inc., the nation's third-largest cable operator, has decided to cash in its investment by putting its system on the selling block. And the offering prospectus circulating among potential bidders discloses just how fast the picture can improve in cable - and how attractive an operation can be for others in the business who can benefit from economies of scale.

Mayor Koch issued a compendium of anecdotes, homilies, advice and insults yesterday, all stitched together and released in the form of his second book. In the new memoir, ''Politics,'' a sequel to the best-selling ''Mayor,'' Mr. Koch flits back and forth between his days in Congress, his political beginnings as a reform Democrat and the present. He reiterates his discomfort with ''richies'' and impatience with reformers. He tells candid tales about the powerful and even his late father. He uses expletives with abandon, lectures on racism, calls himself an ''anti-hero'' and repeatedly notes his love of combat.

In the face of solid opposition from the Republican leadership in the House of Representatives, President Reagan ''strongly'' urged the House to pass tax-revision legislation next week and send the bill to the Senate. The President did not wholeheartedly embrace the sweeping bill approved Tuesday by the House Ways and Means Committee, but he said House passage was the only way to keep the tax issue alive in Congress. Democratic supporters of the committee's bill said the House would probably pass the measure if the President reinforced his statement with an all-out effort to round up votes for it. Comment From an Aide Asked whether the President would do that, a White House spokesman said, ''The President will do what he thinks is necessary to keep the process going.''

President Reagan named Vice Adm. John M. Poindexter today to be the White House national security adviser, in a move the President said would allow ''continuity'' in foreign policy and national security issues. Admiral Poindexter, a career Navy officer who has been the deputy national security adviser since 1983, will replace Robert C. McFarlane. Mr. Reagan, flanked by both men in the White House press briefing room, announced Mr. McFarlane's departure at the same time that he named Admiral Poindexter to be the replacement.

Deutsche Bank A.G., West Germany's largest bank, plans to buy the country's largest family-held industrial enterprise, the Flick group, the two companies said today. A Deutsche Bank source said the takeover, which was initiated by Flick, would be worth about $1.8 billion, making it the largest takeover in West German history. After the purchase, expected before the end of the year, Deutsche Bank said it would offer shares in the Flick group to the public through a stock offering sometime next year.