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A100997
Indices of primes in sequence defined by A(0) = 91, A(m) = 10*A(m-1) - 9 for m > 0.
1
2, 3, 4, 8, 21, 26, 35, 56, 61, 77, 200, 536, 695, 789, 904, 1037, 66885, 70499, 91835, 100612, 127239, 380733, 583695
OFFSET
1,1
COMMENTS
Numbers k such that 90*10^k + 1 is prime.
Numbers k such that digit 9 followed by k >= 0 occurrences of digit 0 followed by digit 1 is prime.
Numbers corresponding to terms <= 1037 are certified primes.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A056797(n) - 1.
EXAMPLE
900001 is prime, hence 4 is a term.
PROG
(PARI) a=91; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-9)
(PARI) for(n=0, 1500, if(isprime(90*10^n+1), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 26 2004
EXTENSIONS
66885 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(18)-a(21) from Kamada data by Ray Chandler, Apr 28 2015
a(22)-a(23) from Kamada data by Mohammed Yaseen, Jul 20 2021
STATUS
approved