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 A283209 Primes of the form 299...977...7 with at least one 9 and one 7. 1
 299777, 299977, 29999777, 299999977, 2999999777, 299999999777, 2999977777777, 299999999999977, 2999999999977777777, 2999999999999999977, 299999999999977777777, 299999999999999999977, 2999999999999999777777 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If the number of 7's modulo 3 equals 1, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 3. If the number of 9's modulo 6 equals 5, the corresponding 29..97..7 term cannot be in sequence because 299999 and 999999 are divisible by 7. If the number of 7's and the number of 9's are both odd, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 11. LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..868 MATHEMATICA Sort@ Select[Map[FromDigits@ Join[{2}, ConstantArray[9, #1], ConstantArray[7, #2]] & @@ # &, Select[Tuples[Range@ 20, 2], Times @@ Boole@ Map[OddQ, #] == 0 &]], PrimeQ] (* Michael De Vlieger, Mar 06 2017 *) PROG (PARI) do(n)=my(v=List(), p=29, q); for(d=3, n, p=10*p+7; q=p; forstep(i=d-3, 1, -1, if(ispseudoprime(q+=2*10^i), listput(v, q)))); Vec(v) \\ Charles R Greathouse IV, Mar 06 2017 CROSSREFS Sequence in context: A022246 A227700 A050257 * A113939 A023686 A250506 Adjacent sequences: A283206 A283207 A283208 * A283210 A283211 A283212 KEYWORD nonn,base AUTHOR FUNG Cheok Yin, Mar 03 2017 EXTENSIONS More terms from Charles R Greathouse IV, Mar 06 2017 STATUS approved

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Last modified March 2 01:29 EST 2024. Contains 370447 sequences. (Running on oeis4.)