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A077781 Palindromic wing primes (also known as near-repdigit palindromes) of the form 7*(10^a(n)-1)/9-3*10^[ a(n)/2 ]. 1
5, 7, 13, 47, 73, 139, 1123, 1447, 6877, 8209, 18041, 27955, 39311, 64801 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Prime versus probable prime status and proofs are given in the author's table.

a(15) > 2*10^5. - Robert Price, Nov 23 2015

REFERENCES

C. Caldwell and H. Dubner, The near repdigit primes A(n-k-1)B(1)A(k), especially 9(n-k-1)8(1)9(k), Journal of Recreational Mathematics, Volume 28, No. 1, 1996-97, pp. 1-9.

LINKS

Table of n, a(n) for n=1..14.

Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)

Makoto Kamada, Prime numbers of the form 77...77477...77

Index entries for primes involving repunits.

FORMULA

a(n) = 2*A183179(n)+1.

EXAMPLE

a(n)=7 -> 7*(10^7-1)/9-3*10^3 = 7774777.

MATHEMATICA

Do[ If[ PrimeQ[(7*10^n - 27*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 39400, 2}] (* Robert G. Wilson v, Dec 16 2005 *)

PROG

(PARI) for(n=1, 1e3, if(ispseudoprime((7*10^(2*n+1)-27*10^n-7)/9), print1(2*n+1, ", "))) \\ Altug Alkan, Nov 23 2015

CROSSREFS

Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.

Sequence in context: A064600 A174874 A109904 * A102872 A102873 A158892

Adjacent sequences:  A077778 A077779 A077780 * A077782 A077783 A077784

KEYWORD

more,nonn,base

AUTHOR

Patrick De Geest, Nov 16 2002

EXTENSIONS

a(14) from Robert Price, Nov 23 2015

STATUS

approved

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Last modified September 23 21:16 EDT 2017. Contains 292391 sequences.