A062094 a(1) = 2, 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).
2, 12, 34, 217, 731, 1743, 2183, 3759, 21179, 121179, 931303, 1931303, 11175573, 11379829, 17896361, 18419721, 96919009, 889910891, 1889910891, 2742368917, 25741106537, 110203233579, 679231622473, 1679231622473, 7921256096487
Offset: 1
Examples
a(1) = 2 is a prime hence a(2) = 12; a(3) = 34, 3*4 = 12 and 3<4.
Links
- Sean A. Irvine, Table of n, a(n) for n = 1..250
Programs
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Mathematica
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, 2, 25]
Extensions
More terms from Robert G. Wilson v, Aug 08 2001