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A077777 Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form 7*(10^a(n)-1)/9 - 5*10^[ a(n)/2 ]. 1
3, 7, 15, 21, 25, 961, 1899, 3891, 15097, 17847 (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(11) > 2*10^5. - Robert Price, Nov 02 2015

REFERENCES

C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.

LINKS

Table of n, a(n) for n=1..10.

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

Makoto Kamada, Prime numbers of the form 77...77277...77

Index entries for primes involving repunits.

FORMULA

a(n) = 2*A183178(n)+1.

EXAMPLE

a(n)=15 -> 7*(10^15-1)/9-5*10^7 = 777777727777777.

MATHEMATICA

Do[ If[ PrimeQ[(7*10^n - 45*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 1000, 2}] (* Robert G. Wilson v, Dec 16 2005 *)

CROSSREFS

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

Sequence in context: A144751 A138847 A235699 * A153829 A153830 A080550

Adjacent sequences:  A077774 A077775 A077776 * A077778 A077779 A077780

KEYWORD

more,nonn,base

AUTHOR

Patrick De Geest, Nov 16 2002

STATUS

approved

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Last modified September 23 20:33 EDT 2017. Contains 292391 sequences.