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A000355 Primes = 3, 9, 11 (mod 20) such that 2p+1 is also prime. 3
3, 11, 23, 29, 83, 89, 131, 191, 251, 431, 443, 491, 509, 683, 743, 809, 911, 1031, 1049, 1103, 1223, 1229, 1289, 1409, 1451, 1511, 1583, 1811, 1889, 1931, 2003, 2063, 2069, 2129, 2351, 2543, 2549, 2903, 2963, 2969, 3023, 3329, 3389, 3449, 3491, 3623, 3803 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) = (A000353(n)-1)/2. - Reinhard Zumkeller, Feb 10 2009

REFERENCES

R. A. J. Matthews, Maximally periodic reciprocals, Bull. Institute of Mathematics and Its Applications, vol. 28, p. 147-148, 1992.

LINKS

R. Zumkeller, Table of n, a(n) for n = 1..1000

MATHEMATICA

Select[Prime[Range[1000]], MatchQ[Mod[#, 20], 3|9|11] && PrimeQ[2#+1]&] (* Jean-Fran├žois Alcover, Feb 07 2016 *)

PROG

(PARI) is(n)=my(k=n%20); (k==3||k==9||k==11) && isprime(2*n+1) && isprime(n) \\ Charles R Greathouse IV, Nov 20 2014

CROSSREFS

Subset of A005384.

Cf. A000353.

Sequence in context: A177789 A141226 A049491 * A163769 A100860 A018630

Adjacent sequences:  A000352 A000353 A000354 * A000356 A000357 A000358

KEYWORD

nonn,easy

AUTHOR

R. A. J. Matthews [ 100265.3005(AT)compuserve.com ]

EXTENSIONS

More terms from Reinhard Zumkeller, Feb 10 2009

STATUS

approved

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Last modified December 8 11:05 EST 2016. Contains 278939 sequences.