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

11, 19, 31, 59, 61, 79, 101, 109, 131, 139, 151, 179, 181, 199, 211, 229, 241, 269, 271, 349, 401, 409, 421, 439, 461, 479, 491, 499, 521, 569, 571, 599, 601, 619, 631, 659, 661, 709, 751, 769, 811, 829, 881, 919, 941, 1009, 1021, 1039, 1051, 1069, 1091, 1109

Offset: 1

Author

Giovanni Teofilatto, May 11 2016

Keywords

• nonn
• easy
• base

Comments

a(n) + a(n+1) = 0 (mod 10) for all n >= 1.

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..10000

Programs

Maple

a:= proc(n) option remember; local p, d;
      if n=1 then p:= 11
    else p:= a(n-1); d:= `if`(n::odd, 1, 9);
         while irem(p, 10)<>d do p:=nextprime(p) od
      fi; p
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, May 11 2016

Mathematica

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 *)