OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
46 and 49 are two consecutive semiprimes with the same multiplicative digital root, namely 8.
MATHEMATICA
mrt[{a_, b_}]:=NestWhile[Times@@IntegerDigits[#]&, a, #>9&]== NestWhile[ Times@@IntegerDigits[#]&, b, #>9&]; With[{semis=Partition[Select[Range[ 600], PrimeOmega[#]==2&], 2, 1]}, Transpose[Select[semis, mrt]][[1]]] (* Harvey P. Dale, Dec 17 2011 *)
PROG
(PARI) A031347(n)= { local(resul, ncpy); if(n<10, return(n) ); ncpy=n; resul = ncpy % 10; ncpy = (ncpy - ncpy%10)/10; while( ncpy > 0, resul *= ncpy %10; ncpy = (ncpy - ncpy%10)/10; ); return(A031347(resul)); } { oldr=0; oldn=0; for(n=5, 1000, if( bigomega(n)==2, dr=A031347(n); if(dr==oldr, print1(oldn, ", "); ); oldr=dr; oldn=n; ); ); } \\ R. J. Mathar, May 23 2006
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), May 20 2006
EXTENSIONS
Corrected by R. J. Mathar, May 23 2006
STATUS
approved