# CoVID-19 SEIR model implemented in HP 35S RPN
# optimized further
# changes suggested by Manolis Christodoulou
C001 LBL C
C002 SF 11 # SF .1 to get SF 11
C003 (EQN) A+B+C+D+E+F+G+H+I+J # Press EQN (invisible), then RCL letter, then +
# get nsteps from tmax over dt
C004 RCL J
C005 RCL/ I
C006 STO K #nsteps
# calculate dS
C007 RCL H
C008 RCL* F
C009 RCL* A
C010 RCL* C
C011 RCL* I
C012 +/-
C013 STO L #dS
# calculate dE
C014 RCL H
C015 RCL* F
C016 RCL* A
C017 RCL* C
C018 RCL E
C019 RCL* B
C020 -
C021 RCL* I
C022 STO M #dE
# calculate dI
C023 RCL E
C024 RCL* B
C025 RCL G
C026 RCL* C
C027 -
C028 RCL* I
C029 STO N #dI
# calculate dR
C030 RCL G
C031 RCL* C
C032 RCL* I
C033 STO O #dR
# calculate new S, E, I, and R
C034 RCL L
C035 STO+ A #new S
C036 RCL M
C037 STO+ B #new E
C038 RCL N 
C039 STO+ C #new I
C040 RCL O
C041 STO+ D #new R
# continue for nsteps (K)
C042 DSE K 
C043 GTO C007
C044 RTN