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A069687
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Primes that yield another prime on placing a 1 on both sides (as leading and trailing digits).
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11
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3, 5, 17, 23, 29, 47, 53, 83, 107, 113, 131, 149, 173, 197, 239, 251, 317, 359, 383, 401, 443, 503, 509, 599, 641, 683, 701, 719, 743, 797, 821, 947, 953, 1031, 1049, 1103, 1109, 1187, 1229, 1277, 1283, 1301, 1373, 1583, 1613, 1619, 1637, 1733, 1847, 1889
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OFFSET
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1,1
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LINKS
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EXAMPLE
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239 belongs to this sequence as 12391 is also a prime.
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MAPLE
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a:= proc(n) option remember; local p;
p:= `if`(n=1, 1, a(n-1));
do p:= nextprime(p);
if isprime(parse(cat(1, p, 1))) then break fi
od; p
end:
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MATHEMATICA
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Select[ Range[2000], PrimeQ[ # ] && PrimeQ[ FromDigits[ Insert[ IntegerDigits[ # ], 1, {{1}, {-1}}]]] &]
san[i_]:=1+Prime[i]*10+10^(Floor@Log[10, Prime[i]]+2); Prime@Select[Range[1000], PrimeQ@san[#]&] (* From José María Grau Ribas, Jan 22 2012 *)
Select[Prime[Range[300]], PrimeQ[FromDigits[Join[{1}, IntegerDigits[#], {1}]]]&] (* Harvey P. Dale, May 18 2012 *)
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PROG
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(PARI) forprime( p=1, 9999, isprime(10^#Str(p*10)+p*10+1) & print1(p", ")) \\ M. F. Hasler, May 18 2012
(PARI) A069687_vec(Nmax=10^4)=my(p, d=1); vector(Nmax, i, until(isprime((d+p)*10+1), d<(p=nextprime(p+1))&d*=10); p) \\ M. F. Hasler, May 19 2012
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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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Corrected and edited following suggestions by H. P. Dale and others by M. F. Hasler, May 18 2012
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STATUS
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approved
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