Citigroup has urged investors to buy takeover target AstraZeneca after it turned down rival Pfizer's latest bid.Citi has added AstraZeneca to its Citi Focus List Europe, saying it is worth £49 a share following its rejection of Pfizer's "final" £55 per share offer.AstraZeneca's shares endured a record intra-day decline after its decision and were trading 54.5p off at 4233p by 10:38 in London.Pfizer had little scope to change its mind and make a higher offer before a deadline of next Monday due to strict takeover rules, analysts said.But Citi highlighted its report in November saying AstraZeneca's line-up of early to mid-stage oncology drugs was extremely under-valued.It also noted that its 2023 revenue forecast of $31bn, against a consensus of $28bn, was still materially below AstraZeneca's internal forecast of $45bn."We therefore add 'buy' rated AstraZeneca to the Citi Focus List based on our extensive previous analysis of its fundamental value drivers," the broker said.PW