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 A107124 Numbers n such that (10^(2n+1)+27*10^n-1)/9 is prime. 3
 2, 3, 32, 45, 1544 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS n is in the sequence iff the palindromic number 1(n).4.1(n) is prime (dot between numbers means concatenation). If n is in the sequence then n is not of the forms 3m+1, 16m+11, 16m+12, 18m+11, 18m+15, etc. (the proof is easy). REFERENCES C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9. LINKS Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's) Makoto Kamada, Prime numbers of the form 11...11411...11 FORMULA a(n) = (A077780(n)-1)/2. EXAMPLE 32 is in the sequence because the palindromic number (10^(2*32+1)+27*10^32-1)/9 = 1(32).4.1(32) = 11111111111111111111111111111111411111111111111111111111111111111 is prime. MATHEMATICA Do[If[PrimeQ[(10^(2n + 1) + 27*10^n - 1)/9], Print[n]], {n, 2200}] Select[Range[1600], PrimeQ[FromDigits[Join[PadRight[{}, #, 1], {4}, PadRight[ {}, #, 1]]]]&] (* Harvey P. Dale, Aug 01 2017 *) PROG (PARI) is(n)=ispseudoprime((10^(2*n+1)+27*10^n-1)/9) \\ Charles R Greathouse IV, May 22 2017 CROSSREFS Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187. Sequence in context: A041053 A212763 A103108 * A141049 A052830 A041895 Adjacent sequences:  A107121 A107122 A107123 * A107125 A107126 A107127 KEYWORD nonn,base,more AUTHOR Farideh Firoozbakht, May 19 2005 EXTENSIONS Edited by Ray Chandler, Dec 28 2010 STATUS approved

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