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%I #19 Jul 10 2023 10:04:53
%S 0,1,2,3,4,14,18,19,45,51,52,191,379,587,775,905,1349,1735,2913,7507,
%T 15709,16452,17487,18108,21603,25890,26724,34837,67467
%N Indices of primes in sequence defined by A(0) = 97, A(n) = 10*A(n-1) - 63 for n > 0.
%C Numbers n such that 90*10^n + 7 is prime.
%C Numbers n such that digit 9 followed by n >= 0 occurrences of digit 0 followed by digit 7 is prime.
%C Numbers corresponding to terms <= 1349 are certified primes.
%C a(30) > 2*10^5. - _Robert Price_, Oct 16 2015
%D Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/90007.htm#prime">Prime numbers of the form 900...007</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A096774(n) - 1.
%e 90007 is prime, hence 3 is a term.
%t Select[Range[0, 200000], PrimeQ[90*10^# + 7] &] (* _Robert Price_, Oct 16 2015 *)
%o (PARI) a=97;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-63)
%o (PARI) for(n=0,1500,if(isprime(90*10^n+7),print1(n,",")))
%Y Cf. A000533, A002275, A096774.
%K nonn,hard,more
%O 1,3
%A _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(25)-a(29) from Kamada data by _Ray Chandler_, Apr 28 2015
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