HP 40gs User Manual page 193

Computer Algebra System (CAS)
Typing:
DEF(U(N) = 2N+1)
produces the result:
U(N) = 2N+1
Typing:
U(3)
then returns:
7
Example
Calculate the first six Fermat numbers F1...F6 and
determine whether they are prime.
So, you want to calculate:
k
2
F k ( )
= 2 + 1
Typing the formula:
2
2
2 + 1
gives a result of 17. You can then invoke the
ISPRIME?() command, which is found in the MATH
key's Integer menu. The response is 1, which means
TRUE. Using the history (which you access by pressing the
SYMB key), you put the expression
Equation Writer with ECHO, and change it to:
3
2
2 + 1
Or better, define a function F(K) by selecting DEF from the
ALGB menu on the menu bar and type:
( ( )
DEF F K = 2
2
2
The response is
variables (which you can verify using the VARS key).
For K=5, you then type:
F(5)
for k = 1...6
2
k
2
)
+ 1
k
+ 1
and F is now listed amongst the
2
2
+ 1
into the
14-11