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A156770
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1 followed by least greater integer such that concatenation of a(n-1) and a(n) is prime.
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2
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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
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OFFSET
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1,2
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LINKS
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EXAMPLE
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The term immediately after 17 is 21 because 1721 is the first prime greater than 1717.
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MAPLE
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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
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MATHEMATICA
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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 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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