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%I M2783 #20 Mar 22 2024 19:38:18
%S 1,3,9,21,363,2161,4839,49521,105994,207777
%N Numbers k such that k*10^k + 1 is prime.
%C These are generalized Cullen numbers in base 10. - Julien Peter Benney (jpbenney(AT)ftml.net), Oct 24 2004
%C No others less than 270026. - _Ray Chandler_, Apr 10 2016
%D J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 363, p. 84, Ellipses, Paris 2008.
%D H. Dubner, Generalized Cullen numbers, J. Rec. Math., 21 (No. 3, 1989), 190-191.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Daniel Hermle, <a href="http://www.primzahlenarchiv.de/primes10.html">First Coordinated Generalized Cullen Prime Search</a>.
%H Guenter Loeh, <a href="http://guenter.loeh.name/gc/status.html">Generalized Cullen primes</a>.
%e For k = 3 we get (3*10^3)+1 = (3*1000)+1 = 3000 + 1 = 3001, which is prime.
%e For k = 9 we get 9*10^9+1 = 9*1000000000+1 = 9000000000+1 = 9000000001, which is prime.
%o (PARI) is(n)=ispseudoprime(n*10^n+1) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y Cf. A004023.
%K hard,nonn
%O 1,2
%A _N. J. A. Sloane_
%E More terms from Julien Peter Benney (jpbenney(AT)ftml.net), Jun 11 2005
%E Edited by _N. J. A. Sloane_ at the suggestion of _Andrew S. Plewe_, Jun 05 2007
%E a(10) from Loeh link by _Ray Chandler_, Apr 10 2016
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