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A183180 Numbers n such that (7*10^(2n+1) - 18*10^n - 7)/9 is prime. 1
0, 1, 7, 13, 58, 129, 253, 1657, 2244, 2437, 7924, 9903, 11899, 18157, 18957, 23665 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

a(16) > 10^5. - Robert Price, Jun 23 2017

0 could be considered part of this sequence since the formula evaluates to 5 which is a degenerate form of the near-repdigit palindrome 777...77577...777 with 0 occurrences of the digit 7. - Robert Price, Jun 23 2017

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..16.

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

Makoto Kamada, Prime numbers of the form 77...77577...77

Index entries for primes involving repunits.

FORMULA

a(n) = (A077785(n) - 1)/2.

MATHEMATICA

Do[If[PrimeQ[(7*10^(2n + 1) - 18*10^n - 7)/9], Print[n]], {n, 3000}]

PROG

(PARI) is(n)=ispseudoprime((7*10^(2*n+1)-18*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017

CROSSREFS

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

Sequence in context: A015441 A255286 A253210 * A133664 A143794 A159198

Adjacent sequences:  A183177 A183178 A183179 * A183181 A183182 A183183

KEYWORD

nonn,base

AUTHOR

Ray Chandler, Dec 28 2010

EXTENSIONS

a(15) from Robert Price, Jun 23 2017

STATUS

approved

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