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A056263
Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) + 27 for n > 0.
1
1, 3, 27, 155, 321, 351, 1211, 1283, 7983, 15191, 84771, 119929, 148859
OFFSET
1,2
COMMENTS
Numbers n such that (720*10^n - 27)/9 is a prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 9 followed by digit 7 is a prime.
Numbers corresponding to terms <= 1283 are certified primes. For larger numbers see P. De Geest, PDP Reference Table.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A082716(n) - 2.
EXAMPLE
797 is a prime, hence 1 is a term.
PROG
(PARI) a=77; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+27)
(PARI) for(n=0, 1500, if(isprime((720*10^n-27)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert G. Wilson v, Aug 18 2000
EXTENSIONS
Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, Jun 08 2007
Update from De Geest link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(11)=84771 from Ray Chandler, Jan 04 2011
a(12)=119929 from Ray Chandler, Apr 01 2011
a(13)=148859 from Ray Chandler, Apr 09 2011
Updated comments section by Patrick De Geest, Nov 02 2014
Edited by Ray Chandler, Nov 04 2014
STATUS
approved