In[86]:= Reduce[NextPrime[x] - x == 2 && 0 < x < 20 && PrimeQ[x], x \[Element] Integers] Out[86]= False In[87]:= Reduce[NextPrime[x] - x == 2 && 0 < x < 20, x \[Element] Integers] Out[87]= x == 3 || x == 5 || x == 9 || x == 11 || x == 15 || x == 17
why Out[86]=False ?
I expect
Out[86]= x == 3 || x == 5 ||x == 11 ||x == 17
Gianluca, your are simply too fast - and with the better solution! Best regards -- Henrik
I suppose it is because PrimeQ is meant as a test for numbers. When given a symbol, such as in PrimeQ[x], it gives False.
PrimeQ
PrimeQ[x]
False
I would use Element[x, Primes], which remains unevaluated when x is a symbol.
Element[x, Primes]
x
