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 A242990 Numbers n such that concatenating 1 with five instances of n produces a prime. 4
 9, 21, 41, 101, 143, 171, 249, 251, 297, 329, 333, 347, 357, 489, 509, 531, 537, 557, 569, 611, 629, 653, 657, 669, 731, 773, 821, 843, 861, 941, 957, 983, 993, 1133, 1143, 1209, 1229, 1239, 1313, 1361, 1409, 1427, 1451, 1497, 1539, 1647, 1673, 1791, 1833 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Jens Kruse Andersen, Table of n, a(n) for n = 1..10000 EXAMPLE 143 is included because 1143143143143143 is a prime. MATHEMATICA cQ[n_, i_]:=Module[{idn=IntegerDigits[n]}, PrimeQ[FromDigits[Flatten[Join[ {1}, Table[ idn, {i}]]]]]]; Select[Range[2000], cQ[#, 5]&] PROG (Python) from sympy import isprime for n in range(2*10**3): ..if isprime(int('1'+5*str(n))): ....print(n, end=', ') # Derek Orr, Aug 17 2014 (PARI) s=[]; for(n=1, 10^4, d=length(Str(n)); if(isprime(10^(5*d)+(10^(5*d)-1)/(10^d-1)*n), s=concat(s, n))); s \\ Jens Kruse Andersen, Aug 18 2014 CROSSREFS Cf. A242987, A242988, A242989. Sequence in context: A107890 A110209 A184040 * A053476 A110680 A163205 Adjacent sequences: A242987 A242988 A242989 * A242991 A242992 A242993 KEYWORD nonn,base,easy AUTHOR Harvey P. Dale, Aug 17 2014 STATUS approved

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Last modified March 30 06:16 EDT 2023. Contains 361606 sequences. (Running on oeis4.)