cp's OEIS Frontend

A291487 a(n) is the smallest k such that psi(k) = n!, or 0 if no such k exists (psi(k) = A001615(k)).

%I A291487 #30 Dec 30 2020 03:41:46
%S A291487 1,1,0,4,12,75,300,1950,13650,122850,1160250,13340250,140390250,
%T A291487 1825073250,25318743450,370489869750,5503458610650,93558796381050,
%U A291487 1643961707838450,30815473745606550,596477734382780250,12526032422038385250,272871020017346000250,6276033460398958005750
%N A291487 a(n) is the smallest k such that psi(k) = n!, or 0 if no such k exists (psi(k) = A001615(k)).
%e A291487 a(5) = 75 because psi(75) = 120 = 5! and 75 is the least number with this property.
%e A291487 a(7) = 1950 and 1950 has no prime factor 7, so a(8) = 7*1950 = 13650.
%o A291487 (PARI) a001615(n) = my(f=factor(n)); prod(i=1, #f~, f[i, 1]^f[i, 2] + f[i, 1]^(f[i, 2]-1))
%o A291487 a(n) = my(N=n!); for(k=1, N, if(a001615(k)==N, return(k))); 0 \\ after _Charles R Greathouse IV_ at A001615
%Y A291487 Cf. A000142, A001615.
%K A291487 nonn
%O A291487 0,4
%A A291487 _Altug Alkan_, Aug 24 2017
%E A291487 a(13)-a(14) from _Giovanni Resta_, Aug 25 2017
%E A291487 a(15)-a(23) from _Daniel Suteu_, Dec 29 2020