A156770 1 followed by least greater integer such that concatenation of a(n-1) and a(n) is prime.

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

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