OFFSET
1,1
COMMENTS
Surprisingly there are only three(?) semiprimes sp, 10,14,15, such that sp minus its digits is semiprime.
That is because n - (sum of its digits) = A066568(n) is divisible by 9. - Robert Israel, Oct 01 2024
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
15=3*5 and 15+1+5=21=3*7.
MAPLE
filter:= n -> numtheory:-bigomega(n) = 2 and numtheory:-bigomega(n+convert(convert(n, base, 10), `+`))=2:
select(filter, [$1..1000]); # Robert Israel, Oct 01 2024
MATHEMATICA
Select[Range[500], Plus@@Last/@FactorInteger[ # ]==Plus@@Last/@FactorInteger[ #+Plus@@IntegerDigits[ # ]]==2&]
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Zak Seidov, Jun 14 2005
STATUS
approved