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%I #15 Oct 19 2017 10:43:03
%S 3,5,211049,253679,41887255409,73768891456259
%N Smaller of the first twin pair to yield a twin pair chain of length n.
%C PARI program written by _Rick L. Shepherd_.
%C If n is in the sequence and n>5 then Mod[n,30]=29. - _Farideh Firoozbakht_, Mar 26 2004
%D a(3) and a(4) found by _Rick L. Shepherd_ and _Don Reble_; a(5) by _David W. Wilson_; a(6) by _Don Reble_.
%e 5,7 form a twin pair, separated by the number 6. Their sum 12 is the separator of another twin pair, 11,13. _Their_ sum is not the separator of a further twin pair. (5,7),(11,13) is a chain of length 2. 5 is the lower of the smallest twin pair to generate a chain of this length, so a(2)=5.
%e Here are the actual chains (lesser of twin primes given): 3; 5 -> 11; 211049 -> 422099 -> 844199; 253679 -> 507359 -> 1014719 -> 2029439; 41887255409 -> 83774510819 -> 167549021639 -> 335098043279 -> 670196086559; ...
%o (PARI) for(n=1,40000,if(prime(n+1)-prime(n)==2, s=prime(n+1)+prime(n); if(isprime(s-1)&&isprime(s+1), if(isprime(2*s-1)&&isprime(2*s+1),print(n,", ", prime(n))))))
%Y Cf. A069142. Equals A069179 - 1.
%K nonn
%O 1,1
%A _Neil Fernandez_, Apr 10 2002
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