OFFSET
1,1
COMMENTS
a(n) == 1 (mod 10).
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
a(2) = 51 because 51^2+1 = 2*1301, 52^2+1 = 5*541, 53^2+1 = 10*281.
MAPLE
with(numtheory):for n from 1 to 30000 do : x:=n^2+1:y:=(n+1)^2+1:z:=(n+2)^2+1:x1:=factorset(x):y1:=factorset(y):z1:=factorset(z):n1:=bigomega(x):n2:=bigomega(y):n3:=bigomega(z):if
x1[1]=2 and n1=2 and y1[1]=5 and n2 = 2 and z1[1]=2 and z1[2]=5 and n3=3 then
printf(`%d, `, n):else fi:od:
MATHEMATICA
ksQ[k_]:=And@@PrimeQ[{(k^2+1)/2, ((k+1)^2+1)/5, ((k+2)^2+1)/10}]; Select[ Range[30000], ksQ] (* Harvey P. Dale, Sep 01 2013 *)
PROG
(PARI) forstep(k=1, 1e5, 10, if(isprime(k^2\2+1)&isprime((k+1)^2\5+1)&isprime((k+2)^2\10+1), print1(k", ")))
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jan 31 2011
STATUS
approved