OFFSET
1,2
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..1000
EXAMPLE
183 is in the sequence because 183^2 + 1 = 2*5*17*197 and 2 = 1^2 + 1, 5 = 2^2+1, 17 = 4^2+1 and 197 = 14^2 + 1.
MAPLE
with(numtheory):nn:=1000:for n from 1 to nn do: x:=n^2+1:y:=factorset(x):ny:=nops(y):id:=0:for
q from 1 to ny do: z:=y[q]-1:zz:=sqrt(z):if zz=floor(zz) then id:=id+1:else fi:od:if id=ny then printf(`%d, `, n):else fi:od:
MATHEMATICA
Select[Range@330, And @@ IntegerQ /@ Sqrt[FactorInteger[#^2 + 1][[All, 1]] - 1] &] (* Ivan Neretin, Aug 31 2016 *)
PROG
(PARI) isok(n) = {fn = factor(n^2+1)[, 1]; for (k=1, #fn, if (!issquare(fn[k]-1), return (0)); ); 1; } \\ Michel Marcus, Sep 01 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jan 29 2011
STATUS
approved