Highest Number of Consecutive Primes

[Sam Benry]

Euler published the remarkable quadratic formula:

n� + n + 41

It turns out that the formula will produce 40 primes for the consecutive values n = 0 to 39. However, when n = 40, 402 + 40 + 41 = 40(40 + 1) + 41 is divisible by 41, and certainly when n = 41, 41� + 41 + 41 is clearly divisible by 41.

Using computers, the incredible formula n� − 79n + 1601 was discovered, which produces 80 primes for the consecutive values n = 0 to 79. The product of the coefficients, −79 and 1601, is −126479.

Considering quadratics of the form:

n� + an + b, where |a| < 1000 and |b| < 1000

where |n| is the modulus/absolute value of n
e.g. |11| = 11 and |−4| = 4

Find the product of the coefficients, a and b, for the quadratic expression that produces the maximum number of primes for consecutive values of n, starting with n = 0.

This is weird, the code I wrote is very simple, it gives correct answers for a = 1 b = 41 and a=-79 b = 1601
but when I run it to find the largest number of consecutive primes, I get 1011 consecutive prime numbers when a = -999 and b = 61
making the product -60939
but this is turning out to be the wrong answer...
anyone can find me an error in the code?

public class Problem27 
{
public static boolean isPrime(long nbr)
{
boolean isNPrime=true;
for (long i = 2; i <= nbr/2; i++)
if ((nbr != i) && (nbr % i == 0))
{
isNPrime = false;
break;
}
if (isNPrime == true) return true;
else return false;
}
public static long producePrime(int a,int b)
{
long n=0;
while(true)
{
if (isPrime(n*n + a*n + b))
n++;
else
break;
}
return n;
}
public static void main(String[] args) 
{
long consecutivePrimes =0;
long temp;
long product=0;
for(int a=-999;a<1000;a++)
for(int b=-999;b<1000;b++)
{
temp = producePrime(a,b);
if (temp>consecutivePrimes)
{
consecutivePrimes=temp;
product=a*b;
}
}
System.out.println(consecutivePrimes);
System.out.println(product);

}
}

[Jim Yingst]

Well, look at the "prime" numbers generated by the formula when a = -999 and b = 61. What do you get when n = 0? n = 1? n = 2? Are all those numbers prime? If not, how can you modify the isPrime() method to correctly identify whether a number is prime or not?