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 Minimal Primes (Posted on 2012-04-24)
409 is an interesting prime number. If you remove any number of its digits, then you will get a number that is not prime.

4=2^2
0=not prime
9=3^2
40=2^3*5
49=7^2
09=9=3^2

Call a number with this property a minimal prime. Find all minimal primes.

 No Solution Yet Submitted by Math Man Rating: 4.6667 (3 votes)

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 of course it also inspires a computer program | Comment 3 of 5 |

This tests whether an input number is a minimal prime, and includes the single-digit primes:

10     repeat
20       input Tst:Tst=cutspc(str(Tst))
30       Pr=""
40       Good=1
50       if prmdiv(val(tst))<val(tst) then good=0:else gosub *Testing(1)
60       if Good then print "good":else print "bad"
65     until tst="0"
70     end
80
90       *Testing(Psn)
100       if Psn=len(Tst) then
110         :Prtst=val(Pr)
120         :if prmdiv(Prtst)=Prtst and Prtst<>0 and Prtst<>1 then Good=0:endif
130       :else
140         :gosub *Testing(Psn+1)
150       Pr=Pr+mid(Tst,Psn,1)
160       if Psn=len(Tst) and Psn>len(Pr) then
170         :Prtst=val(Pr)
180         :if prmdiv(Prtst)=Prtst and Prtst<>0 and Prtst<>1 then Good=0:endif
190       if Psn<len(Tst) then
200         :gosub *Testing(Psn+1)
205       pr=left(pr,len(pr)-1)
210       return

 Posted by Charlie on 2012-04-24 16:50:53

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