Everyone can agree that 1+1=2. But the idea that 7 is greater than 13 -- that some numbers are luckier than others -- makes no sense to some people. Such numerical biases¹ can cause deep divisions.

And that is what happened earlier this month in Hong Kong. Property developer Henderson Land Development Co. made news for selling a condominium for $56.6 million, a price the developer called a residential² record in Asia. But after that sale was announced, the property began making news for other unusual numbers. Henderson is labeling the floors of its property at 39 Conduit Road with numbers that increase, but not in the conventional 1-then-2 way. The floor above 39, for example, is 60. And the top three floors are consecutively³ labeled 66, 68 and 88.

This offended some people's sense of order. At a protest Sunday against high housing prices, Hong Kong Democratic Party legislators expressed dissatisfaction with the numbering scheme's tenuous⁴ relationship to reality. 'You could call the ground floor the 88th floor, but it's meaningless,' says Emily Lau. 'When you say you live on the 88th floor, people expect you to be on the 88th floor, not the 10th floor or something.'

Numerology, a belief that certain digits⁵ have greater meaning beyond merely their quantity, has long been been viewed as a kind of loony uncle to mathematics. Numerologists favor or fear certain numbers depending on factors such as the sound of the words for those numbers or the letter in the alphabet they correspond to. That kind of reasoning leads some mathematicians⁷, who are governed by numerical laws and properties, to believe they have one up on numerologists.

But many mathematicians have their own emotional attachments⁸ to numbers that drove them to enter the field in the first place. Some will cop to having numerical crushes that might not look that different from numerologists'.

'The idea that numbers are somehow pure and immune to superstitious⁹ thinking, because they're somehow more 'objective' than words, doesn't take into account the fact that every concept exists (in our minds) in an interconnected tapestry¹⁰ of emotionally and culturally charged signifiers,' Golan Levin, designer of the interactive¹¹ project The Secret Lives of Numbers, which tracks the popularity of every whole number between one and one million, writes in an email. He considers most numerical superstitions¹² harmless.

Thomas Garrity, a mathematician⁶ at Williams College, has always had a particular fondness for the number 9. The number 51, however, doesn't make his favorites list.

'This might stem from childhood, when I regularly thought that 51 should be prime, even though 51=3x17,' he says, taking a trip down mathematical memory lane. But he doesn't base decisions on his preferences, for instance by avoiding the 51st floor of buildings, he says. 'I can understand people having slightly irrational¹³ feelings about particular numbers,' Prof. Garrity says. 'I don't get, though, people making real decisions based on such feelings.'

And yet some numerical superstitions do spread, especially when profits are involved. A Las Vegas casino that caters¹⁵ to Hong Kong high rollers also skips floors from 40 to 59, while Henderson's Hong Kong development omits the 13th floor to cater¹⁴ to Western tastes.

A Henderson spokeswoman says customers 'don't want the fours and the unlucky numbers. These numbers are more interesting.'

Henderson chose to name the floors as it did because of positive associations with 6 and 8, and negative ones with 4. In Cantonese and Mandarin¹⁶, the word for eight sounds like 'faat,' which means prosperity. Hence the Beijing Olympics starting time of 8 p.m. on Aug. 8, 2008. The word for four, meanwhile, 'sounds very much like 'death,' and is therefore avoided at all costs,' says Hung-Hsi Wu, professor emeritus¹⁷ of mathematics at University of California, Berkeley, who was born in Hong Kong. Six is also considered lucky.

A preference for six over four also guided developers of the 42-floor Mandalay Bay casino in Las Vegas. There, penthouses are on the 60th, 61st and 62nd floor because Mandalay Bay skips the numbers 40 to 59.

Gordon Absher, spokesman for Mandalay owner MGM Mirage¹⁸, says that decision was shaped by possible perceptions of high rollers when they are assigned to those floors. 'You could think that we are trying to, as the casino, give you bad luck,' Mr. Absher says.

Similarly, developers who would assuage¹⁹ fears of 13 can't avoid the existence of a 13th floor in buildings with 13 or more stories. But they can rename it out of existence. When a 13th floor was added to the Skirvin Hotel in Oklahoma City, in the 1930s, it was named the 14th floor. The hotel was shuttered in 1988 and reopened and renamed in 2007 by Hilton, which nonetheless kept the name for the top floor.

The 22-story headquarters of Chicago-based Marc Realty avoids throwing off the numbers in higher floors by labeling the 13th floor '14A.' It labels the 14th floor '14B.'

'That arrangement keeps the elevations²⁰ of the upper floors straight in a physical sense,' says Marc marketing²¹ coordinator²² Dan Krc. He adds that triskaidekaphobia, or fear of 13, appears to be fading, with floors labeled 13 in Marc properties showing occupancy rates are no lower than other floors.

The negative associations with 13 have been traced to the number of diners at the Last Supper, before the betrayal of Jesus. Some believed it went back to prehistoric²³ times -- the lowest number that couldn't be counted on ten fingers and two feet. (Apparently, individual toes couldn't be counted).

But Underwood Dudley, retired²⁴ professor of mathematics at Depauw University and author of 'Numerology,' says he wasn't able to verify any of these. 'As far as I can tell, some number had to be unlucky, and it was 13,' Dr. Dudley says.

Beverly Kay, a numerologist in Mequon, Wisc., doesn't buy fears of 13. However, she says her work reading meaning into clients' birth dates and names is consistent with math. 'This is scientific,' Ms. Kay says.

Psychologists and historians generally have tied such beliefs to the broader human tendency to seek patterns and systems where none exist. At its extreme, an emotional relationship to a number can creep into obsessive-compulsive behavior. In his book 'Strange Brains and Genius,' Clifford Pickover dug through case studies of numerical obsessive-compulsive disorder²⁵, and found that it could be tied to just about any numeral. Electricity pioneer Nikola Tesla demanded precisely²⁶ 18 clean towels a day and showed an intense preference for multiples of three.

While mathematicians generally don't go to Tesla-like extremes, they possess a generally positive outlook about all numbers and that distinguishes them from numerologists, they say.

For example, Kenneth Ribet, a professor of mathematics at Berkeley, considers some prime numbers 'friends,' he says. One is 144,169, which reads like 12 squared followed by 13 squared; another the easily remembered number of 1,234,567,891.

'Mathematicians don't have numbers that they're afraid of or shy away from because we do really like all of the numbers,' says Prof. Ribet. 'On the other hand, some of us have favorites.'

人人都可以相信，1加1等于2。但7比13好──有些数字比其他数字吉利──对一些人来说就没有道理了。这种数字上的好恶，可能会造成深刻的分歧。

上月早些时候，香港就曾发生过这样一种情况。房地产开发商恒基兆业地产有限公司(Henderson Land Development Co.，简称恒基地产)一套复式公寓卖出5660万美元的天价，轰动一时。恒基地产说，这个数字创了亚洲住宅价格的纪录。这一销售结果公布过后，整套物业又因为其他引人瞩目的数字，再次引起轰动。在给这套位于干德道39号的楼盘编制楼层号码时，恒基没有遵循传统的一二三递增的方式。比如，39楼上面就是 60楼。最顶三层的号码，分别是66楼，68楼和88楼。

在拉斯维加斯，像7这样的幸运数字很受欢迎。图为2007年7月7日，很多情侣在曼德勒海湾酒店排队等待举行婚礼。这侵犯了一些人的秩序感。周日（10月 25日）抗议高房价的过程中，一些香港民主党立法会议员对这种牵强附会的编号方案表达了不满。刘慧卿说，你可以把底楼叫做88楼，但没有意义。你说你住 88楼，人们都以为你住88楼，而不是10楼什么的。

数字命理学(Numerology)相信，某些数字在表示数量之外，还有更多的意义。很久以来，这种信念都被数学家打入另册。根据数字的读音或字母表中跟数字对应的字母，数字命理学家对某些数字的态度或褒或贬。这样一种逻辑，让头脑中只有数理规律与属性的数学家们觉得，自己要比数字命理学家高明。

但很多数学家对于起初促使他们进入这个领域的那些数字，也存在一些爱憎。一些人也会出现数理逻辑上的矛盾，跟数字命理学家们的错误比起来，差别也不是那么大。

数字的秘密人生》(The Secret Lives of Numbers)设计者列文(Golan Levin)在电子邮件里写道，有人认为，数字多少是纯粹的、与迷信思维无涉，因为相比语词，它们总要“客观”些，但这样的观念没有考虑到，每一个观念，都是以富含情感与文化的各种信号所织成的锦缎的形式存在于我们的大脑中的。《数字的秘密人生》是一个互动项目，跟踪1到100万之间每一个整数的受欢迎程度。

威廉姆斯学院(Williams College)数学家嘉里蒂(Thomas Garrity)对数字9一直有着一种特殊的喜爱。但数字51却不在他的喜好之列。

他回忆自己的数学之路说，这可能跟我小时候有关，当时我经常以为51应该是质数，虽然51=3×17。但他说，自己不会以自己的好恶来做决定，比如说是避开大楼里的第51楼。嘉里蒂教授说，我可以理解人们对某些数字的一些细微的非理性情感，不过我不能理解人们以这样的情感为基础来做真正的决定。

但某些数字迷信还是被扩大化，特别是在涉及利润的时候。当香港恒基为迎合西式口味省掉13楼时，拉斯维加斯一家赌场为迎合香港赌客，楼层编号也略过了40到59楼。

恒基一位女发言人说，客户不喜欢带4的数字和不吉利的数字。这些数字有着更为丰富的蕴涵。

恒基所以按照它的方式编制楼层号码，是因为6和8能引起积极联想，4会引起消极联想。加州大学伯克利分校(University of California, Berkeley)名誉数学教授伍鸿熙(Hung-Hsi Wu)说，在粤语和普通话中，8读起来像发财的“发”。因为这个原因，北京奥运会开幕选在了2008年8月8日的晚上8点。而数字4听起来很像“死”，所以人们会不顾一切地避免。6也被认为是吉利数字。

对6的偏爱和对4的排斥，影响了拉斯维加斯42层楼高的赌场曼德勒湾 (Mandalay Bay)的开发商。这里的顶层都被命名为60层，61层和62层，因为它略过了40到59。

曼德勒湾业主美高梅金殿梦幻(MGM Mirage)发言人阿布谢(Gordon Absher)表示，这种决定缘于赌客被安排到这些楼层时可能有的一些观念。他说，你可能会觉得，我们作为赌场想给你带来厄运。

同样，希望减轻人们对13的恐惧的开发商，不能够避免一幢楼层数目不低于13的大楼里存在第13楼。但他们可以通过命名来让“13”消失。上世纪30年代，俄克拉荷马城(Oklahoma City)的Skirvin酒店加盖第13层的时候，这层楼被命名为14楼。这座酒店在1988年停止营业，2007年被希尔顿(Hilton)收购后重新开张和命名，不过顶层的编号仍然没有变化。

芝加哥(Chicago)22层楼的Marc Realty则把第13层楼命名为“14A”，把第14层命名为“14B”。这样就没有让上面的楼层名不副实。

Marc营销协调人克尔奇(Dan Krc)说，这种安排让上面的楼层高度以一种直观的方式呈现。他说，“13恐惧症”似乎正在消失，Marc物业中被标为13楼的楼层，入住率并不比其他楼层低。

数字13引起的负面联想，源自于耶稣遭背叛前夕“最后的晚餐”当中的就餐者人数。而一些人相信，这种联想要追溯到史前时代──13是不能用十指或两只脚数尽的最小数目（显然脚趾头不算）。

但迪普奥大学 (Depauw University) 退休数学教授、《数字命理学》作者达德利(Underwood Dudley)表示，他无法对这些说法予以证实。达德利博士说，据我所知，必须有某个数字是不吉利的，而这个数字就是13。

美国威斯康星州梅库恩(Mequon, Wisc.)的数字命理学家凯(Beverly Kay)并不害怕13。不过她说，她为顾客的生日和姓名赋予意义的工作，与数学是相通的。凯女士说，这是科学。

心理学家和历史学家一般都试图把这种信念跟人类的一种普遍倾向联系起来，这种倾向就是在不存在类型和系统的地方寻找类型和系统。在极端情况下，数字的情感联想可能会潜入强迫性的行为。在《数字的异想世界》(Strange Brains and Genius)一书中，皮寇弗(Clifford Pickover)深入研究了数字性强迫性紊乱的各种案例，发现这种紊乱可以跟任何数字发生联系。电子学先驱台斯拉(Nikola Tesla)每天要用18块干净毛巾，并对3的倍数显示了强烈的偏好。

数学家一般不会达到台斯拉这样的极端，他们认为，他们对所有数字是一视同仁的，而这把他们和数字命理学家们区别开来。

例如加州大学伯克利分校教授里贝(Kenneth Ribet)就说，他把一些质数视为“朋友”。其中一个是144,169，就像是12的平方跟了一个13的平方。另外一个比较容易记住的数字是1,234,567,891。

里贝教授说，数学家并没有怕见到或不愿见到的数字，因为我们对所有数字真的都很喜欢。他说，另一方面，我们当中一些人确实偏爱一些数字。