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Background: In the laws of niddah there is something called a vesst haflaga, which is a period that occurs at some consistent interval. If a woman establishes a pattern for some amount of consecutive periods, there are various halachic stringencies and leniencies that result from assuming that this pattern will continue.

The most straightforward occurrence of such a pattern would be if a woman had her period at equidistant times, i.e. day 1, 31, 61, 91 (30 days apart). However, it can follow more complicated patterns, such as an incrementally increasing pattern (1, 30, 60, 91 ...) (S.A. Y.D. 189:5). The pattern can also consist of a repeating series, such as 25 - 26 - 27 - 25 - 26 - 27 - 25 - 26 - 27, in which the pattern of 25 days later, followed by 26 days later, followed by 27 days later is repeated 3 times, and that whole series becomes the assumed pattern (Y.D. 189:8).

My question is, how complicated can such a pattern be? Can it be a Fibonacci Sequence? Can it be ascending perfect squares? Do the numbers need to be mappable by a polynomial function (as opposed to, say, the set of prime numbers*)?

*I'm not so interested in debating the mathematical claim

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    How about any OEIS sequence? – Double AA Jun 8 '15 at 20:25
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    Saw this question on Twitter, thought that it might be something about a vest (and misspelled, at that)...glad to see I've been proven wrong :) – MTL Jun 8 '15 at 23:35
  • It seems based off the Shulcahn Aruch that this is the most complicated case that counts. The Shulchan Aruch could have given more complex examples and didnt so it seems like tthats it. – sam Jun 9 '15 at 0:04
  • I found interesting the question. But lets say that can be Fibonacci, set of prime numbers, perfect squares... I believe that you wont find ANY woman in the world with these patterns – juanora Jun 10 '15 at 3:04
  • @juanora You are most likely correct, and it doesn't change the value of the question. – Y     e     z Jun 10 '15 at 3:21
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The Badei Hashulchan 189:5 (שבכל ענין) asks this question. His case is where it gets bigger by an extra day each time. He remains with the question, צ"ע.

EDIT: As for Fibonacci, the Mechaber in 189:10 has a case of 15,16,18 and says that since the last one jumped one more there is no vesst. The Shach there says that this would be the case even if there was a vesst kavua on the 14th beforehand.

This shows us that we do not set a vesst of a Fibonacci pattern, since we now have 14,15,16,18. According to the Fibonacci pattern she should be choshesh on the 21st of the next month.

  • +1 - I actually saw the badei hashulchan yesterday and was going to post it, but first I am trying to figure out how he understands the shach there which seems to address the question. – Y     e     z Jun 11 '15 at 19:28
  • But I think you made a mistake for Fibonacci - there aren't enough periods in that sequence to be kove'a. Even with the 14th, as the Badei HaShulchan points out, you would need 5 periods for such a sequence. (And I think you meant 21st of next month) – Y     e     z Jun 11 '15 at 19:29
  • @yEz First of all thanks for sending me there in the place. Isn't the Shach dealing with a steady 2 ahead (30,32,34,36)? – HaLeiVi Jun 11 '15 at 19:42
  • No - the shach is coming to give an example of a non-שוה vesst. Meaning, as opposed to the case in the S.A. of increasing by a constant 1, it is increasing by 1, then 2. The Shach is being מדייק on the S.A.'s words שבכל ענין שתהא שוה. Many Rishonim actually have those words. – Y     e     z Jun 11 '15 at 19:45
  • ( I assume you are looking at shach 189:12) – Y     e     z Jun 11 '15 at 19:46

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