The People’s Republic of China (PRC) is engaged in a wide‑ranging and heavily resourced campaign to acquire sensitive United States technologies and intellectual property, in part to bolster the modernization and capability of its military, the People’s Liberation Army (PLA). The PRC’s acquisition of sensitive United States technologies and intellectual property to modernize its military is a threat to our Nation’s long-term economic vitality and the safety and security of the American people.
The PRC authorities use some Chinese students, mostly post‑graduate students and post-doctorate researchers, to operate as non-traditional collectors of intellectual property. Thus, students or researchers from the PRC studying or researching beyond the undergraduate level who are or have been associated with the PLA are at high risk of being exploited or co-opted by the PRC authorities and provide particular cause for concern. In light of the above, I have determined that the entry of certain nationals of the PRC seeking to enter the United States pursuant to an F or J visa to study or conduct research in the United States would be detrimental to the interests of the United States.
NOW, THEREFORE, I, DONALD J. TRUMP, President of the United States, by the authority vested in me by the Constitution and the laws of the United States of America, including sections 212(f) and 215(a) of the Immigration and Nationality Act (INA), 8 U.S.C. 1182(f) and 1185(a), and section 301 of title 3, United States Code, hereby find that the unrestricted entry into the United States as nonimmigrants of persons described in section 1 of this proclamation would, except as provided for in section 2 of this proclamation, be detrimental to the interests of the United States, and that their entry should be subject to certain restrictions, limitations, and exceptions. I therefore hereby proclaim the following:
现在，我是美国总统唐纳德·特鲁姆（DONALD J. TRUMP），由美国《宪法》和美国法律（包括移民法第212（f）和215（a）条）赋予我的权力和《国籍法》（INA），8 USC据此，《美国法典》第1182（f）和1185（a）条以及第3篇第301条由此认定，本条第1节所述的人作为非移民者不受限制地进入美国，除非本条第1节规定本公告的第2条不利于美国的利益，并且其加入应受到某些限制，限制和例外。因此，我在此宣布以下内容：
Section 1. Suspension and Limitation on Entry. The entry into the United States as a nonimmigrant of any national of the PRC seeking to enter the United States pursuant to an F or J visa to study or conduct research in the United States, except for a student seeking to pursue undergraduate study, and who either receives funding from or who currently is employed by, studies at, or conducts research at or on behalf of, or has been employed by, studied at, or conducted research at or on behalf of, an entity in the PRC that implements or supports the PRC’s “military-civil fusion strategy” is hereby suspended and limited subject to section 2 of this proclamation. For the purposes of this proclamation, the term “military-civil fusion strategy” means actions by or at the behest of the PRC to acquire and divert foreign technologies, specifically critical and emerging technologies, to incorporate into and advance the PRC’s military capabilities.
Sec. 2. Scope of Suspension and Limitation on Entry. (a) Section 1 of this proclamation shall not apply to:
(i) any lawful permanent resident of the United States;
(ii) any alien who is the spouse of a United States citizen or lawful permanent resident;
(iii) any alien who is a member of the United States Armed Forces and any alien who is a spouse or child of a member of the United States Armed Forces;
(iv) any alien whose travel falls within the scope of section 11 of the United Nations Headquarters Agreement or who would otherwise be allowed entry into the United States pursuant to United States obligations under applicable international agreements;
(v) any alien who is studying or conducting research in a field involving information that would not contribute to the PRC’s military‑civil fusion strategy, as determined by the Secretary of State and the Secretary of Homeland Security, in consultation with the appropriate executive departments and agencies (agencies);
(vi) any alien whose entry would further important United States law enforcement objectives, as determined by the Secretary of State, the Secretary of Homeland Security, or their respective designees, based on a recommendation of the Attorney General or his designee; or
(vii) any alien whose entry would be in the national interest, as determined by the Secretary of State, the Secretary of Homeland Security, or their respective designees.
(b) Nothing in this proclamation shall be construed to limit the ability of an individual to seek asylum, refugee status, withholding of removal, or protection under the Convention Against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, consistent with the laws of the United States.
Sec. 3. Implementation and Enforcement. (a) Persons covered by sections 1 or 2 of this proclamation shall be identified by the Secretary of State or the Secretary of State’s designee, in his or her sole discretion, pursuant to such standards and procedures as the Secretary of State may establish. For purposes of subsections 2(a)(v), 2(a)(vi), and 2(a)(vii) of this proclamation, the Secretary of State shall provide for identifications of aliens based on the further determinations and recommendations provided for in those subsections by the Attorney General and the Secretary of Homeland Security.
(b) The Secretary of State shall implement this proclamation as it applies to visas pursuant to such procedures as the Secretary of State, in consultation with the Secretary of Homeland Security, may establish in the Secretary of State’s discretion. The Secretary of Homeland Security shall implement this proclamation as it applies to the entry of aliens pursuant to such procedures as the Secretary of Homeland Security, in consultation with the Secretary of State, may establish in the Secretary of Homeland Security’s discretion.
(c) An alien who circumvents the application of this proclamation through fraud, willful misrepresentation of a material fact, or illegal entry shall be a priority for removal by the Department of Homeland Security.
Sec. 4. Termination. This proclamation shall remain in effect until terminated by the President. The Secretary of State, in consultation with the Secretary of Homeland Security, may at any time recommend that the President continue, modify, or terminate this proclamation.
Sec. 5. Effective Date. This proclamation is effective at 12:00 p.m. eastern daylight time on June 1, 2020.
Sec. 6. Additional Measures. (a) The Secretary of State shall consider, in the Secretary’s discretion, whether nationals of the PRC currently in the United States pursuant to F or J visas and who otherwise meet the criteria described in section 1 of this proclamation should have their visas revoked pursuant to section 221(i) of the INA, 8 U.S.C. 1201(i).
(b) Within 60 days of the effective date of this proclamation, the Secretary of State and the Secretary of Homeland Security, in consultation with the heads of appropriate agencies, shall review nonimmigrant and immigrant programs and shall recommend to the President, through the Assistant to the President for National Security Affairs, any other measures requiring Presidential action that would mitigate the risk posed by the PRC’s acquisition of sensitive United States technologies and intellectual property.
(c) The Secretary of State and the Secretary of Homeland Security shall, within the scope of their respective authorities and in coordination with the heads of appropriate agencies, take action to further mitigate the risk posed by the PRC’s acquisition of sensitive United States technologies and intellectual property. The Secretary of State and the Secretary of Homeland Security shall report to the President, within 60 days of the effective date of this proclamation, through the Assistant to the President for National Security Affairs, any such planned and executed actions.
(d) The Secretary of State and the Secretary of Homeland Security shall consider issuing updated regulations and guidance, as appropriate, implementing the inadmissibility provisions in section 212(a)(3)(D) of the INA, 8 U.S.C. 1182(a)(3)(D).
Sec. 7. General Provisions. (a) Nothing in this proclamation shall be construed to impair or otherwise affect:
(i) the authority granted by law to an executive department or agency, or the head thereof; or
(ii) the functions of the Director of the Office of Management and Budget relating to budgetary, administrative, or legislative proposals.
(b) This proclamation shall be implemented consistent with applicable law and subject to the availability of appropriations.
(c) This proclamation is not intended to, and does not, create any right or benefit, substantive or procedural, enforceable at law or in equity by any party against the United States, its departments, agencies, or entities, its officers, employees, or agents, or any other person.
IN WITNESS WHEREOF, I have hereunto set my hand this twenty-ninth day of May, in the year of our Lord two thousand twenty, and of the Independence of the United States of America the two hundred and forty-fourth.
这个问题的应用场景十分广泛。例如：对于 Google 主页面而言，同一个账户可能会访问 Google 主页面多次。于是，在诸多的访问流水中，如何计算出 Google 主页面每天被多少个不同的账户访问过就是一个重要的问题。那么对于 Google 这种访问量巨大的网页而言，其实统计出有十亿 的访问量或者十亿零十万的访问量其实是没有太多的区别的，因此，在这种业务场景下，为了节省成本，其实可以只计算出一个大概的值，而没有必要计算出精准的值。
Count-distinct Problem 的维基百科：https://en.wikipedia.org/wiki/Count-distinct_problem
Heule, Stefan, Marc Nunkesser, and Alexander Hall. “HyperLogLog in practice: algorithmic engineering of a state of the art cardinality estimation algorithm.” Proceedings of the 16th International Conference on Extending Database Technology. 2013.
Flajolet, Philippe, et al. “Hyperloglog: the analysis of a near-optimal cardinality estimation algorithm.” 2007.
在上图中，我们可以看到论文中使用的数据都具有某种周期性（Periodicity）。KPI A，B，C 都是具有明显具有工作日和周末特点的，在工作日和周末分别有着不同的形状；KPI D 则是关于网上应用商店周五促销的，因此在周五周六的时候，其实时间序列会出现一个尖峰（peak）；KPI E 的话则是每隔 7 天，会有两个尖刺，然后并且迅速恢复；KPI F 的话则是可以看出时间序列在十一的走势跟其余的时间点明显有区别。除此之外，对于一些做旅游，电商等行业的公司，其节假日效应会更加突出一点，而且不同的业务在节假日的表现其实也是不一样的。有的时间序列在节假日当天可能会上涨（电商销售额），有的时间序列在节假日当天反而会下降（订车票，飞机票的订单量）。因此，在对这些时间序列做异常检测的同时，如何避免其节假日效应就是一个关键的问题了。
从上图可以看到 Period 的核心思路（core idea）。在本文使用的数据中，时间序列的长度较长，一般来说都是好几个月到半年不等，甚至更长的时间。对于一条时间序列（a given KPI），可以将它的历史数据（historical data）进行按天切分，获得多个子序列（sub KPIs）。对于这多个子序列，需要进行聚类以得到不同类别。或者按照日历直接把时间序列的工作日（work day），休息日（off day），春节（spring festival）序列进行切分，将工作日放在一起，休息日放在一起，春节放在一起。把这些子序列进行拼接就可以得到三条时间序列数据，分别是原时间序列的工作日序列（work day subsequence），休息日序列（off day subsequence），春节序列（spring festival subsequence）。然后分别对着三条时间序列训练一个异常检测的模型（例如 Holt-Winters 算法，简写为 HW）。对于新来的时间序列，可以根据当日具体的日期（工作日，休息日或者春节）放入相应的模型进行异常检测，从而进一步地得到最终的结果。