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A101961
Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 41 for n > 0.
1
1, 2, 4, 5, 11, 185, 434, 1745, 3446, 18797, 70208
OFFSET
1,2
COMMENTS
Numbers n such that (230*10^n - 41)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.
Numbers corresponding to terms <= 434 are certified primes.
a(12) > 10^5. - Robert Price, Mar 04 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A098930(n) - 1.
EXAMPLE
255551 is prime, hence 4 is a term.
PROG
(PARI) a=21; for(n=0, 1800, if(isprime(a), print1(n, ", ")); a=10*a+41)
(PARI) for(n=0, 1800, if(isprime((230*10^n-41)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
EXTENSIONS
3446 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(10)-a(11) from Robert Price, Mar 04 2015
STATUS
approved