OFFSET
1,2
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..10000
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A002496} p/(p-1) = Product_{k in A005574} (1 + 1/k^2) = 2.809865... - Amiram Eldar, Sep 27 2020
EXAMPLE
a(17) = 74 because 74 = 2*37 = (1^2+1)*(6^2+1).
MAPLE
with(numtheory):T:=array(1..50):U:=array(1..1000):k:=1:for m from 1 to 300
do:x:=m^2+1:if type(x, prime)=true then T[k]:=x:k:=k+1:else fi:od:for x from
2 to 2000 do: B:=factorset(x):yy:=nops(B):A:=convert(T, set):if A intersect
B = B then printf(`%d, `, x):else fi:od:
MATHEMATICA
Select[Range@520, And @@ IntegerQ /@ Sqrt[FactorInteger[#][[All, 1]] - 1] &] (* Ivan Neretin, Aug 31 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jan 20 2011
STATUS
approved