Is cross-posted forbidden/discouraged on Stack Exchange sites? If so, why?

Question

Math people:

I have never posted a question to Mathematics meta before and I hope this is the right thing to do. Yesterday I posted a question to Math Stack Exchange (is there a name for this function (parity of a finite sequence)?). I received a good answer last night, but not 100% of what I wanted. I posted a question to Computational Science Beta to try to get an answer to the remainder of what I wanted (I included a link to my original question at Math Stack Exchange), and I got a good answer. I posted a similar question to Stack Overflow lasta night and got an even better answer. Today I showed the improved answer to the respondent at Computational Science Beta. He told me today (but not last night) that cross-posting was discouraged.

Is this true? If so, why? I got distinct, useful answers from the three sites. I have cross-posted a few times in the past and no one complained. Once, I posted a numerical-methods type question at Math Stack Exchange, and I got a comment that I would get better answers at Computational Science Beta, so I posted it there, and I actually did get better answers there.

Regarding the question I posted last night, I could have broken it up into a math question and a computing question, posted the math question on Math Stack Exchange and the computing question on Computational Science Beta.

If I cross-post occasionally, word my questions so that they don't look the same, and don't tell anyone at any site about the posts on the other Stack Exchanges, is anyone likely to care enough to hunt me down and find out what I have done? I have enough experience at mathematics and computing to know when both math people and computational science people might be able to help me with a question.

I just spotted a Similar Question that may help me, and I apologize for not reading it first. But I will submit my question in case it has any particulars that may help you answer it.

More specifics: original math+computing question: is there a name for this function (parity of a finite sequence)?

(received good answer to math part of question)

unresolved computing question: https://scicomp.stackexchange.com/questions/7053/how-can-i-compute-whether-a-sequence-is-an-even-or-odd-permutation-of-an-increas

(received good answer to computing part of question)

more focused computing question I formulated after attempting to answer computing question myself: https://stackoverflow.com/questions/16351039/is-there-a-built-in-function-in-matlab-to-determine-if-a-permutation-is-even-or

(received better answer than the one I got at Computational Science Beta)

Stefan (Stack Exchange FAN)

Answer 1

My personal opinion is that cross-posting is okay as long as each question links to all other versions of itself. Otherwise you risk unnecessary duplication of effort (someone slaves over a hot stove to bring you an answer on one site only to find that someone else has already written the same stuff on another site), which I think is mildly disrespectful to the answerers.

Answer 2

In addition to the point made by Qiaochu Yuan about wasted effort on duplicate answers, crossposting without crosslinking also does a disservice to anyone else who may later have the same problem and tries to search for it, since the answers will be scattered across multiple sites.

In order to make them easier to find, I have edited the three questions you mentioned above to link to each other. Should you wish to crosspost any questions across multiple Stack Exchange (or other) sites, I would ask you to please include such a notice (either directly in your question, or at least as a comment) from the beginning, in order to address both concerns.

(I must say that I have occasionally wished for some way to allow true crossposting across Stack Exchange sites, i.e. the ability to have a single question appear on multiple sites. Having a question migrated from one site to another does something almost like this, but it does have a couple of distinctly inconvenient aspects — not the least being that it's not really meant to be used for deliberate crossposting.)