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A063380 a(1) = 4, a(n) = concatenation of two closest factors of a(n-1) whose product equals a(n-1) or if a(n-1) is a prime then the concatenation of 1 and a(n-1).

%I A063380 #2 Mar 30 2012 17:30:32
%S A063380 4,22,211,1211,7173,9797,97101,910789,1182799,1319029,6719687,7678761,
%T A063380 32559587,257126691,1591617149,6653239233,62767105999,126149775659,
%U A063380 432933715713,2435717774509,6598336914323,19495633384521
%N A063380 a(1) = 4, a(n) = concatenation of two closest factors of a(n-1) whose product equals a(n-1) or if a(n-1) is a prime then the concatenation of 1 and a(n-1).
%t A063380 f[ n_Integer ] := (d = Divisors[ n ]; l = Length[ d ]; If[ EvenQ[ l ], ToExpression[ ToString[ d[ [ l/2 ] ] ] <> ToString[ d[ [ l/2 + 1 ] ] ] ], ToExpression[ ToString[ d[ [ l/2 + .5 ] ] ] <> ToString[ d[ [ l/2 + .5 ] ] ] ] ] ); NestList[ f, 4, 24 ]
%Y A063380 Cf. A003681, A062094 and A062095.
%K A063380 base,nonn
%O A063380 1,1
%A A063380 _Robert G. Wilson v_, Aug 08 2001