OFFSET
1,1
EXAMPLE
a(5)=21 and it is neither prime nor practical. It is the 5th such occurrence.
MATHEMATICA
PracticalQ[n_] := Module[{f, p, e, prod = 1, ok = True}, If[n<1||(n>1&&OddQ[n]), False, If[n==1, True, f=FactorInteger[n]; {p, e}=Transpose[f]; Do[If[p[[i]]>1+DivisorSigma[1, prod], ok=False; Break[]]; prod=prod*p[[i]]^e[[i]], {i, Length[p]}]; ok]]]; Select[Range[1, 1000], ! PracticalQ[#] && ! PrimeQ[#] &] (* using T. D. Noe's program A005153 *)
PROG
(PARI) isok(n) = bittest(n, 0) && return(n==1); my(P=1); n && !for(i=2, #n=factor(n)~, n[1, i]>1+(P*=sigma(n[1, i-1]^n[2, i-1])) && return) ;
for(n=1, 200, if(!isok(n) && !isprime(n), print1(n, ", "))) \\ Altug Alkan, Nov 19 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Frank M Jackson, Nov 18 2015
STATUS
approved