A106556 Smallest number formed by concatenating the two factors of a semiprime.

22, 23, 33, 25, 27, 35, 37, 112, 55, 132, 113, 172, 57, 192, 133, 223, 77, 173, 115, 193, 229, 231, 135, 233, 237, 117, 241, 175, 243, 293, 137, 313, 247, 195, 253, 337, 235, 259, 177, 1111, 261, 341, 343, 197, 267, 347, 271, 1113, 295, 273, 315, 279, 353

Offset: 1

Author

Eric Angelini, May 09 2005

Keywords

• base
• easy
• nonn

Examples

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

Programs

Mathematica

f[n_]:=Module[{ids=IntegerDigits/@Transpose[FactorInteger[n]][[1]], a,b},a =FromDigits[ Join[ First[ids],Last[ids]]];b=FromDigits[ Join[ Last[ids], First[ids]]];Min[a,b]]; f/@Select[Range[200], PrimeOmega[#] ==2&] (* Harvey P. Dale, Jun 20 2011 *)

Extensions

More terms from Harvey P. Dale, Jun 20 2011