A106557 Largest number that can be obtained by concatenating the two factors of the n-th semiprime.

22, 32, 33, 52, 72, 53, 73, 211, 55, 213, 311, 217, 75, 219, 313, 232, 77, 317, 511, 319, 292, 312, 513, 323, 372, 711, 412, 517, 432, 329, 713, 331, 472, 519, 532, 373, 523, 592, 717, 1111, 612, 413, 433, 719, 672, 473, 712, 1311, 529, 732, 531, 792, 533

Offset

1,1

Links

Andrew Howroyd, Table of n, a(n) for n = 1..1000

Formula

a(n) = A084797(A001358(n)). - Andrew Howroyd, Jan 08 2020

Example

First semiprime is 4; 4 is 2*2 -> 22.
Second semiprime is 6; 6 is 3*2 -> 32 (and not 23).
...
Eighth semiprime is 22; 22 is 2*11 -> 211 (and not 112).

Prog

(PARI) \\ here cd(x, y) returns base 10 concatenation.
cd(v1, v2)={10^(logint(v2, 10) + 1)*v1 + v2}
seq(n)={my(v=vector(n), k=0); for(i=1, #v, k++; while(2<>bigomega(k), k++); my(f=factor(k)[, 1]); v[i] = if(#f==1, cd(f[1], f[1]), max(cd(f[1], f[2]), cd(f[2], f[1])))); v} \\ Andrew Howroyd, Jan 08 2020

Crossrefs

Cf. A001358, A084797, A106556.

Sequence in context: A335348 A179282 A106555 * A142347 A217059 A121019

Adjacent sequences: A106554 A106555 A106556 * A106558 A106559 A106560

Keyword

base,easy,nonn

Author

Eric Angelini, May 09 2005

Extensions

Edited by N. J. A. Sloane, Apr 14 2008

Terms a(22) and beyond from Andrew Howroyd, Jan 08 2020

Status

approved