login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A101062
Indices of primes in sequence defined by A(0) = 83, A(n) = 10*A(n-1) - 7 for n > 0.
1
0, 1, 3, 4, 7, 9, 21, 222, 253, 349, 378, 400, 435, 1153, 1245, 2052, 2686, 3724, 4270, 13095, 21426, 30265, 36790, 41758
OFFSET
1,3
COMMENTS
Numbers n such that (740*10^n + 7)/9 is prime.
Numbers n such that digit 8 followed by n >= 0 occurrences of digit 2 followed by digit 3 is prime.
Numbers corresponding to terms <= 435 are certified primes.
a(25) > 10^5. - Robert Price, Oct 15 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103075(n) - 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
EXAMPLE
822223is prime, hence 4 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(740*10^# + 7)/9] &] (* Robert Price, Oct 15 2015 *)
PROG
(PARI) a=83; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-7)
(PARI) for(n=0, 1500, if(isprime((740*10^n+7)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(20)-a(24) from Kamada data by Ray Chandler, Apr 29 2015
STATUS
approved