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 A074721 Concatenate the primes as 2357111317192329313..., then insert commas from left to right so that between each pair of successive commas is a prime, always making the new prime as small as possible. 4
 2, 3, 5, 7, 11, 13, 17, 19, 2, 3, 2, 93137414347535961677173798389971011031071091131, 2, 7, 13, 11, 3, 7, 13, 91491511, 5, 7, 163, 167, 17, 3, 17, 9181, 19, 11, 9319, 7, 19, 9211223227229233239241251257, 2, 6326927, 127, 7, 2, 81283, 2, 93307, 3, 11, 3, 13, 3, 17, 3, 3, 13, 3, 7, 3, 47, 3, 493533593673733 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Note that leading zeros are dropped. Example: When the primes 691, 701, 709, and 719 get concatenated and digitized, they become {..., 6, 9, 1, 7, 0, 1, 7, 0, 9, 7, 1, 9, ...}. These will end up in A074721 as: a(98)=691, a(99)=7, a(100)=17, a(101)=97, a(102)=19, ..., . Terms a(100) & a(101) have associated with them unstated leading zeros. - Hans Havermann, Jun 26 2009 Large terms in the links are probable primes only. For example, a(1290) has 24744 digits and a(4050), 32676 digits. If of course any probable primes were not actual primes, the indexing of subsequent terms would be altered. - Hans Havermann, Dec 28 2010 What is the next term after {2, 3, 5, 7, 11, 13, 17, 19}, if any, giving a(k)=A000040(k)? LINKS Robert G. Wilson v, Table of n, a(n) for n = 1..329 [a(330) is too large to be included in a b-file: see the a-file) Hans Havermann, Two-color listing of 5359 terms Robert G. Wilson v, Table of n, a(n) for n = 1..1289 MATHEMATICA id = IntegerDigits@ Array[ Prime, 3000] // Flatten; lst = {}; Do[ k = 1; While[ p = FromDigits@ Take[ id, k]; !PrimeQ@p || p == 1, k++ ]; AppendTo[lst, p]; id = Drop[id, k], {n, 1289}] PROG (PARI) a=0; tryd(d) = { a=a*10+d; if(isprime(a), print(a); a=0); } try(p) = { if(p>=10, try(p\10)); tryd(p%10); } forprime(p=2, 1000, try(p)); \\ Jack Brennen, Jun 25 2009 (Haskell) a074721 n = a074721_list !! (n-1) a074721_list = f 0 \$ map toInteger a033308_list where    f c ds'@(d:ds) | a010051'' c == 1 = c : f 0 ds'                   | otherwise = f (10 * c + d) ds -- Reinhard Zumkeller, Mar 11 2014 CROSSREFS Cf. A073034, A047777, A053648, A069090. Cf. A033308. Sequence in context: A171057 A229786 A242122 * A242121 A242120 A259277 Adjacent sequences:  A074718 A074719 A074720 * A074722 A074723 A074724 KEYWORD nonn,base,nice AUTHOR Reinhard Zumkeller, Sep 04 2002 EXTENSIONS Edited by Robert G. Wilson v, Jun 26 2009 Further edited by N. J. A. Sloane, Jun 27 2009, incorporating comments from Leroy Quet, Hans Havermann, Jack Brennen and Franklin T. Adams-Watters STATUS approved

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Last modified August 5 19:14 EDT 2021. Contains 346488 sequences. (Running on oeis4.)