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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 November 18 07:22 EST 2019. Contains 329252 sequences. (Running on oeis4.)