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A028988 Repdigit + 1 is prime. 15
1, 2, 4, 6, 22, 66, 88, 222, 666666, 22222222, 66666666, 666666666, 22222222222, 66666666666, 88888888888888, 88888888888888888, 66666666666666666666, 66666666666666666666666, 88888888888888888888888888888888888 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Next term is 22...2 (36 digits).

LINKS

Table of n, a(n) for n=1..19.

P. De Geest, Repdigits: WONplate 83

MATHEMATICA

a[n_]:=NestList[FromDigits[Append[{#}, n]]&, n, 34]; Union[Join[{1}, Flatten[Table[Select[a[n], PrimeQ[#+1]&], {n, 2, 8, 2}]]]] (* Jayanta Basu, May 30 2013 *)

Select[Flatten[Table[FromDigits[PadRight[{}, n, k]], {n, 40}, {k, 9}]], PrimeQ[ #+1]&] (* Harvey P. Dale, Apr 22 2018 *)

CROSSREFS

Cf. A010785.

Sequence in context: A227626 A152482 A045960 * A172451 A086172 A261832

Adjacent sequences:  A028985 A028986 A028987 * A028989 A028990 A028991

KEYWORD

nonn,base

AUTHOR

Patrick De Geest

EXTENSIONS

Offset corrected by Arkadiusz Wesolowski, Aug 14 2011

STATUS

approved

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Last modified September 22 12:42 EDT 2019. Contains 327307 sequences. (Running on oeis4.)