OFFSET
1,1
COMMENTS
Hardy & Subbarao prove that this sequence is infinite. An upper bound can be extracted from their proof: a(n) < e^e^...^e^O(n log n) with e appearing n times. This tetrational bound could be improved with results on the disjointness of the factorizations of numbers of the form k! + 1. - Charles R Greathouse IV, Sep 15 2015
Named after the Indian mathematician Subbayya Sivasankaranarayana Pillai (1901-1950). - Amiram Eldar, Jun 16 2021
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe)
G. E. Hardy and M. V. Subbarao, A modified problem of Pillai and some related questions, Amer. Math. Monthly, Vol. 109, No. 6 (2002), pp. 554-559; alternative link.
Wikipedia, Pillai prime.
MATHEMATICA
ok[p_] := (r = False; Do[If[Mod[m! + 1, p] == 0 && Mod[p, m] != 1, r = True; Break[]], {m, 2, p}]; r); Select[Prime /@ Range[111], ok] (* Jean-François Alcover, Apr 22 2011 *)
nn=1000; fact=1+Rest[FoldList[Times, 1, Range[nn]]]; t={}; Do[p=Prime[i]; m=2; While[m<p && !(Mod[p, m]!=1 && Mod[fact[[m]], p]==0), m++]; If[m<p, AppendTo[t, p]], {i, 2, PrimePi[nn]}]; t (* T. D. Noe, Apr 22 2011 *)
PROG
(PARI) is(p)=my(t=Mod(5040, p)); for(m=8, p-2, t*=m; if(t==-1 && p%m!=1, return(isprime(p)))); 0 \\ Charles R Greathouse IV, Feb 10 2013
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
R. K. Guy, Sep 08 2001
EXTENSIONS
More terms from David W. Wilson, Sep 08 2001
STATUS
approved