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a(n) is the least semiprime having a(n-1) as a substring, a(1)=4.
1

%I #23 Jul 14 2019 22:21:01

%S 4,14,141,1141,11141,111413,1111413,11114137,111114137,1111141373,

%T 11111141373,111111413737,1111114137373,11111141373734,

%U 1111114137373402,11111141373734023,111111413737340234,11111114137373402349,111111114137373402349,1111111141373734023499

%N a(n) is the least semiprime having a(n-1) as a substring, a(1)=4.

%H Jon E. Schoenfield, <a href="/A254718/b254718.txt">Table of n, a(n) for n = 1..71</a>

%H Jon E. Schoenfield, <a href="/A254718/a254718.txt">Terms and factors for n = 1..71</a>

%H Jon E. Schoenfield, <a href="/A254718/a254718_1.txt">Terms and factors for n = 1..71 and tentative additional terms through n=84</a>

%e 4=2*2, 14=2*7, 141=3*47, 1141=7*163, 11141=13*857, 111413=43*2591, 1111413=3*370471, 11114137=47*236471, 111114137=31*3584327, 1111141373=1051*1057223.

%Y Cf. A001358, A030456.

%K nonn,base

%O 1,1

%A _Zak Seidov_, Feb 06 2015

%E a(11)-a(20) from _Jon E. Schoenfield_, Feb 19 2015