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 A069687 Primes that yield another prime on placing a 1 on both sides (as leading and trailing digits). 9
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE 239 belongs to this sequence as 12391 is also a prime. 947 and 19471 are both primes ==> 947 is in the sequence. [From José María Grau Ribas, Jan 22 2012] MAPLE 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: seq(a(n), n=1..100);  # Alois P. Heinz, May 18 2012 MATHEMATICA 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 *) PROG (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 CROSSREFS Cf. A069688, A069689 & A069690. Sequence in context: A152079 A027699 A153417 * A079017 A211440 A100564 Adjacent sequences:  A069684 A069685 A069686 * A069688 A069689 A069690 KEYWORD nonn,base AUTHOR Amarnath Murthy, Apr 06 2002 EXTENSIONS Edited and extended by Robert G. Wilson v, May 03 2002 Corrected and edited following suggestions by H. P. Dale and others by M. F. Hasler, May 18 2012 STATUS approved

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Last modified August 4 16:34 EDT 2020. Contains 336202 sequences. (Running on oeis4.)