login
A101957
Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 31 for n > 0.
1
1, 2, 5, 10, 29, 127, 182, 374, 2158, 2567, 3368, 24988, 38809, 59557, 74873
OFFSET
1,2
COMMENTS
Numbers n such that (220*10^n - 31)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
Numbers corresponding to terms <= 374 are certified primes.
a(16) > 10^5. - Robert Price, Apr 11 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A102952(n) - 1.
EXAMPLE
2441 is prime, hence 2 is a term.
MATHEMATICA
Select[Range[0, 10000], PrimeQ[(220*10^# - 31)/9] &] (* Robert Price, Apr 11 2015 *)
PROG
(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 1500, if(isprime((220*10^n-31)/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(15) derived from A102952 by Robert Price, Apr 11 2015
STATUS
approved