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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
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.)