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 A156770 1 followed by least greater integer such that concatenation of a(n-1) and a(n) is prime. 2
 1, 3, 7, 9, 11, 17, 21, 29, 39, 43, 49, 51, 53, 81, 91, 99, 103, 123, 127, 133, 153, 191, 227, 231, 241, 249, 253, 273, 281, 291, 293, 311, 323, 333, 337, 339, 341, 347, 359, 377, 387, 397, 427, 429, 431, 441, 443, 453, 461, 467, 471, 481, 489, 493, 523, 541 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Paolo P. Lava, Table of n, a(n) for n = 1..10000 EXAMPLE The term immediately after 17 is 21 because 1721 is the first prime greater than 1717. MAPLE cat2 := proc(a, b) a*10^(max(1, ilog10(b)+1))+b ; end: A156770 := proc(n) option remember ; local a; if n = 1 then 1; else for a from procname(n-1)+1 do if isprime( cat2(procname(n-1), a) ) then RETURN(a) ; fi; od: fi; end: seq(A156770(n), n=1..80) ; # R. J. Mathar, Feb 20 2009 MATHEMATICA nxt[n_]:=Module[{k=n+2, idn=IntegerDigits[n]}, While[!PrimeQ[ FromDigits[ Join[ idn, IntegerDigits[ k]]]], k = k+2]; k]; NestList[nxt, 1, 60] (* Harvey P. Dale, Jul 09 2015 *) PROG (Python) from sympy import isprime from itertools import islice def agen(): an = 1 while True: yield an s, an = str(an), an+1 while not isprime(int(s+str(an))): an += 1 print(list(islice(agen(), 56))) # Michael S. Branicky, Oct 17 2022 CROSSREFS Sequence in context: A275602 A173699 A287202 * A088630 A129747 A354570 Adjacent sequences: A156767 A156768 A156769 * A156771 A156772 A156773 KEYWORD nonn,base AUTHOR Gerald Hillier, Feb 15 2009, Mar 13 2010 EXTENSIONS More terms from R. J. Mathar, Feb 20 2009 STATUS approved

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Last modified February 24 07:07 EST 2024. Contains 370294 sequences. (Running on oeis4.)