This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A069621 #4 May 21 2025 18:57:23 %S A069621 9,97,197,1973,31973,319733,3319733,331973311,6331973311,633197331131, %T A069621 5633197331131,563319733113127,6563319733113127,65633197331131279, %U A069621 3465633197331131279,346563319733113127933,18346563319733113127933 %N A069621 a(1) = 9; a(2n) = smallest prime that is a right concatenation of a(2n-1) and a number with no insignificant zeros and a(2n+1) = smallest prime ending in ( the least significant digits) a(2n). Alternate left and right concatenation yielding primes. %e A069621 a(4) = 1973 starting with a(3) =197 and a(5) = 31973 ending in a(4) = 1973. %Y A069621 Cf. A053582, A069605, A069606, A069607, A069608, A069609, A069610, A069611, A069613, A069614, A069615, A069616, A069617, A069618, A069619, A069620. %K A069621 nonn,base %O A069621 1,1 %A A069621 _Amarnath Murthy_, Mar 27 2002 %E A069621 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 30 2003