OFFSET
1,1
COMMENTS
a(4) is 26 2's followed by 27 1's; a(5) is 33 2's followed by 34 1's.
The sequence is conjectured to be infinite.
Let b(n) be the sequence of corresponding k's. b(1)-b(8) are 1, 2, 3, 26, 33, 215, 259, 799. - Felix Fröhlich, Sep 04 2016
MATHEMATICA
Select[FromDigits@ Join[ConstantArray[2, #], ConstantArray[1, # + 1]] & /@ Range@ 36, PrimeQ] (* Michael De Vlieger, Sep 04 2016 *)
Select[Table[FromDigits[Join[PadRight[{}, n, 2], PadRight[{}, n+1, 1]]], {n, 40}], PrimeQ] (* Harvey P. Dale, Mar 02 2023 *)
PROG
(PARI) a002275(n) = (10^n-1)/9
a011557(n) = 10^n
terms(n) = my(i=0, k=1); while(1, my(x=2*a002275(k)*a011557(k+1)+a002275(k+1)); if(ispseudoprime(x), print1(x, ", "); i++); k++; if(i==n, break))
/* Print initial five terms as follows: */
terms(5) \\ Felix Fröhlich, Sep 04 2016
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Bob Selcoe, Sep 03 2016
STATUS
approved