OFFSET
1,1
COMMENTS
Numbers n such that (220*10^n + 23)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime.
Numbers corresponding to terms <= 179 are certified primes.
With alternating signs, expansion of cosh(atan(x)).
a(15) > 10^5. - Robert Price, Mar 15 2015.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A102954(n) - 1.
EXAMPLE
2447 is prime, hence 2 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(220*10^# + 23)/9] &] (* Robert Price, Mar 14 2015 *)
PROG
(PARI) a=27; for(n=0, 2000, if(isprime(a), print1(n, ", ")); a=10*a-23)
(PARI) for(n=0, 2000, if(isprime((220*10^n+23)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(12)-a(14) derived from A102954 by Robert Price, Mar 15 2015
STATUS
approved