cp's OEIS Frontend

A330293 a(1) = 1, a(2) = 2; for n > 2, a(n) = the smallest prime divisor of the number formed by the concatenation of a(1) to a(n-1) that has not previously appeared in the sequence.

%I A330293 #5 Dec 09 2019 23:25:05
%S A330293 1,2,3,41,7,653,331,2536483,191,176081,18307,2143406938831,101,73,
%T A330293 3541,439,5665417,37,17302849,86113,11,878390431,2969,
%U A330293 1385625388248048145493629820571541645230648738185397486740279040908468652182116663161996667,59,30956837,181,151,159833,1629097816565791058167,293,2063,3251,31219483,13
%N A330293 a(1) = 1, a(2) = 2; for n > 2, a(n) = the smallest prime divisor of the number formed by the concatenation of a(1) to a(n-1) that has not previously appeared in the sequence.
%C A330293 The next term a(36) requires the factorization of a composite 246 digit number 18604...12467.
%e A330293 a(3) = 3 as the concatenation of a(1) and a(2) = '12' and 3 is the smallest prime divisor of 12 that has not appeared in the sequence.
%e A330293 a(4) = 41 as the concatenation of a(1)..a(3) is '123' and 41 is the smallest prime divisor of 123 which has not appeared in the sequence. Note that 3 also divides 123 but a(3) = 3.
%e A330293 a(6) = 653 as the concatenation of a(1)..a(5) is '123417' and 653 is the smallest prime divisor of 123417 has not appeared in the sequence. Note that 9 also divides 123417 and has not appeared but only prime divisors are considered.
%Y A330293 Cf. A020639, A000040, A000005, A330290, A330291, A240588.
%K A330293 nonn,more,hard,base
%O A330293 1,2
%A A330293 _Scott R. Shannon_, Dec 09 2019