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A272550 Lexicographically earliest increasing sequence of primes such that odd-indexed terms have final digit 1 and even-indexed terms have final digit 9.

%I A272550 #41 Dec 10 2019 00:14:53
%S A272550 11,19,31,59,61,79,101,109,131,139,151,179,181,199,211,229,241,269,
%T A272550 271,349,401,409,421,439,461,479,491,499,521,569,571,599,601,619,631,
%U A272550 659,661,709,751,769,811,829,881,919,941,1009,1021,1039,1051,1069,1091,1109
%N A272550 Lexicographically earliest increasing sequence of primes such that odd-indexed terms have final digit 1 and even-indexed terms have final digit 9.
%C A272550 a(n) + a(n+1) = 0 (mod 10) for all n >= 1.
%H A272550 Alois P. Heinz, <a href="/A272550/b272550.txt">Table of n, a(n) for n = 1..10000</a>
%p A272550 a:= proc(n) option remember; local p, d;
%p A272550       if n=1 then p:= 11
%p A272550     else p:= a(n-1); d:= `if`(n::odd, 1, 9);
%p A272550          while irem(p, 10)<>d do p:=nextprime(p) od
%p A272550       fi; p
%p A272550     end:
%p A272550 seq(a(n), n=1..100);  # _Alois P. Heinz_, May 11 2016
%t A272550 a[1] = 11; a[n_] := a[n] = Block[{d, q = a[n-1]}, d=10-Mod[q,10]; While[ Mod[q = NextPrime@ q, 10] != d]; q]; Array[a, 30] (* _Giovanni Resta_, May 11 2016 *)
%K A272550 nonn,easy,base
%O A272550 1,1
%A A272550 _Giovanni Teofilatto_, May 11 2016