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A063382 a(1) = 5, 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 A063382 #2 Mar 30 2012 17:30:32
%S A063382 5,15,35,57,319,1129,11129,31359,310453,1691837,7241691,15094799,
%T A063382 31486929,159198031,1159198031,6203186877,11721529237,88429132553,
%U A063382 129487682919,1228291054211,1394483927247,8800411584567,34329256355023
%N A063382 a(1) = 5, 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 A063382 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, 5, 24 ]
%Y A063382 Cf. A003681, A062094 and A062095.
%K A063382 base,nonn
%O A063382 1,1
%A A063382 _Robert G. Wilson v_, Aug 08 2001