PS see http://en.wikipedia.org/wiki/Cyclic_redundancy_check .
I. On 18 January 2015 at 13:54, Ian Tickle <[email protected]> wrote: > > At the risk of further extending this philosophical (if not etymological) > discussion: in further defence of 'redundancy' I would point out that 'no > longer needed' is not the only meaning of 'redundant', though admittedly it > is the one that most often grabs the headlines! The meaning of 'redundant' > in the context of employment is actually a relatively recent one and > somewhat changed from the original meaning. > > In a scientific context there's a second meaning of 'redundant', and in > fact this one is much closer to the original one. In information theory > the term 'redundant' applies to extra information added to a message being > passed down a transmission line, in order to reduce corruption and loss of > information, i.e. redundancy is absolutely needed to reduce the error > rate. In a crystallographic context the purpose of redundancy, i.e. > measurements over and above those strictly required to obtain a structure, > is also obviously to reduce errors. 'Additional' here clearly does not > necessarily imply 'not needed'. > > 'Redundant' comes from the Latin 're', meaning 'again', and 'unda', > meaning 'wave', from which of course we get 'inundated' and 'undulator', so > 'redundant' means literally 'coming in waves' or 'overflowing'. So we > could say that redundancy is the process of being inundated by data from an > undulator! > > As BR points out 'multiplicity' has long been used to indicate the number > of equivalent reflexions generated by the point-group symmetry (so in PG222 > h00, hk0 and hkl have respectively multiplicites of 1, 2 and 4 for non-zero > hkl). I googled 'reflection multiplicity' and the top hit was > http://pd.chem.ucl.ac.uk/pdnn/symm2/multj.htm . > > Suppose I want to express the following idea: "Redundancy is likely to be > correlated with multiplicity". How do I express that unambiguously if > 'redundancy' is redefined as 'multiplicity'? > > Cheers > > -- Ian > > On 18 January 2015 at 13:12, Bernhard Rupp <[email protected]> wrote: > >> In defense of redundancy: >> >> While the IUCr online dictionary is notably silent about multiplicity, >> the term itself seems >> already oversubscribed and used differently in various crystallographic >> contexts. >> >> (i) Each general or special position in a crystal structure has a >> certain multiplicity, defined by symmetry. >> >> (ii) General reflection multiplicity M is usually is defined by >> reflection symmetry, and >> when reflections are affected by special operations, the resulting >> corresponding >> lower multiplicity because they map onto themselves is accounted for in >> the epsilon factor e. >> >> Btw a useful table of M and e is Iwasaki & Ito Acta Cryst. (1977). A33, >> 227-229 >> >> (iii) In case of Laue patterns, overlap of higher order reflections is >> also called Multiplicity afaik >> (various Helliwell/Moffat et al papers explain this). >> >> So expanding the term multiplicity to include multiple instances of >> measurements of the same reflections >> - while admittedly avoiding the connotation of obsolescence - adds to its >> promiscuous meaning, >> where context becomes part of the definition.... >> >> I abstain from making any suggestions because in the past this has led to >> interesting >> but time-consuming philosophical discourse, proving in general the >> multiplicity of my reflections >> and positions redundant if not obsolete. >> >> Best, BR >> >> -----Original Message----- >> From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of >> Kay Diederichs >> Sent: Sonntag, 18. Januar 2015 09:28 >> To: [email protected] >> Subject: Re: [ccp4bb] Redundancy vs no of frames >> >> Dear Rohit Kumar, >> >> I prefer the term "multiplicity" instead of "redundancy" because the >> latter has a connotation of "not really needed any more". >> >> The relation then is >> >> multiplicity = c * number_of_frames * oscillation_range >> >> where the constant c depends mainly on the space group. >> >> HTH, >> >> Kay >> >> On Sun, 18 Jan 2015 02:35:46 +0530, rohit kumar <[email protected]> >> wrote: >> >> >Dear all, >> > >> >Can anyone tell me how to calculate number of frames from redundancy or >> >vica versa >> > >> >Thank you >> > >> > >
