Task: Pathological floating point problems
C: #include<stdio.h>
#include<gmp.h>

void firstCase(){
	mpf_t a,b,c;
	
	mpf_inits(a,b,c,NULL);
	
	mpf_set_str(a,"0.1",10);
	mpf_set_str(b,"0.2",10);
	mpf_add(c,a,b);
	
	gmp_printf("\n0.1 + 0.2 = %.*Ff",20,c);
}

void pathologicalSeries(){
	int n;
	mpf_t v1, v2, vn, a1, a2, a3, t2, t3, prod;
	
	mpf_inits(v1,v2,vn, a1, a2, a3, t2, t3, prod,NULL);
	
	mpf_set_str(v1,"2",10);
	mpf_set_str(v2,"-4",10);
	mpf_set_str(a1,"111",10);
	mpf_set_str(a2,"1130",10);
	mpf_set_str(a3,"3000",10);
	
	for(n=3;n<=100;n++){
		mpf_div(t2,a2,v2);
		mpf_mul(prod,v1,v2);
		mpf_div(t3,a3,prod);
		mpf_add(vn,a1,t3);
		mpf_sub(vn,vn,t2);
		
		if((n>=3&&n<=8) || n==20 || n==30 || n==50 || n==100){
			gmp_printf("\nv_%d : %.*Ff",n,(n==3)?1:(n>=4&&n<=7)?6:(n==8)?7:(n==20)?16:(n==30)?24:(n==50)?40:78,vn);
		}
		
		mpf_set(v1,v2);
		mpf_set(v2,vn);
	}
}

void healthySeries(){
	int n;
	
	mpf_t num,denom,result;
	mpq_t v1, v2, vn, a1, a2, a3, t2, t3, prod;
	
	mpf_inits(num,denom,result,NULL);
	mpq_inits(v1,v2,vn, a1, a2, a3, t2, t3, prod,NULL);
	
	mpq_set_str(v1,"2",10);
	mpq_set_str(v2,"-4",10);
	mpq_set_str(a1,"111",10);
	mpq_set_str(a2,"1130",10);
	mpq_set_str(a3,"3000",10);
	
	for(n=3;n<=100;n++){
		mpq_div(t2,a2,v2);
		mpq_mul(prod,v1,v2);
		mpq_div(t3,a3,prod);
		mpq_add(vn,a1,t3);
		mpq_sub(vn,vn,t2);
		
		if((n>=3&&n<=8) || n==20 || n==30 || n==50 || n==100){
			mpf_set_z(num,mpq_numref(vn));
			mpf_set_z(denom,mpq_denref(vn));
			mpf_div(result,num,denom);

			gmp_printf("\nv_%d : %.*Ff",n,(n==3)?1:(n>=4&&n<=7)?6:(n==8)?7:(n==20)?16:(n==30)?24:(n==50)?40:78,result);
		}
		
		mpq_set(v1,v2);
		mpq_set(v2,vn);
	}
}

int main()
{	
	mpz_t rangeProd;
	
	firstCase();
	
	printf("\n\nPathological Series : ");
	
	pathologicalSeries();
	
	printf("\n\nNow a bit healthier : ");
	
	healthySeries();

	return 0;
}

Java: import java.math.BigDecimal;
import java.math.RoundingMode;

public class FPProblems {
    public static void wrongConvergence() {
        int[] INDEXES = new int[] { 3, 4, 5, 6, 7, 8, 20, 30, 50, 100 };
        
        
        double[] fpValues = new double[100];
        fpValues[0] = 2.0;
        fpValues[1] = -4.0;
        for (int i = 2; i < fpValues.length; i++) {
            fpValues[i] = 111.0 - 1130.0 / fpValues[i - 1] + 3000.0 / (fpValues[i - 1] * fpValues[i - 2]);
        }
        
        
        BigRational[] brValues = new BigRational[100];
        brValues[0] = BigRational.valueOf(2);
        brValues[1] = BigRational.valueOf(-4);
        for (int i = 2; i < brValues.length; i++) {
            
            BigRational clause2 = BigRational.valueOf(1130).divide(brValues[i - 1]);
            BigRational clause3 = BigRational.valueOf(3000).divide(brValues[i - 1].multiply(brValues[i - 2]));
            brValues[i] = BigRational.valueOf(111).subtract(clause2).add(clause3);
        }
        
        System.out.println("Wrong Convergence Sequence");
        for (int n : INDEXES) {
            BigDecimal value = brValues[n - 1].toBigDecimal(16, RoundingMode.HALF_UP);
            System.out.println("  For index " + n + ", FP value is " + fpValues[n - 1] + ", and rounded BigRational value is " + value.toPlainString());
        }
        
        return;
    }
    
    public static void chaoticBankSociety() {
        System.out.println("Chaotic Bank Society");
        double balance = Math.E - 1.0;
        
        
        BigRational e = BigRational.ONE;
        BigRational d = BigRational.ONE;
        for (int i = 1; i < 1000; i++) {
            d = d.multiply(BigRational.valueOf(i));
            e = e.add(d.reciprocal());
        }
        System.out.println("DEBUG: e=" + e.toBigDecimal(100, RoundingMode.HALF_UP).toPlainString());
        
        
        
        
        BigRational brBalance = e.subtract(BigRational.ONE);
        for (int year = 1; year <= 25; year++) {
            balance = (balance * year) - 1.0;
            brBalance = brBalance.multiply(BigRational.valueOf(year)).subtract(BigRational.ONE);
            BigDecimal bdValue = brBalance.toBigDecimal(16, RoundingMode.HALF_UP);
            System.out.println("  Year=" + year + ", FP balance=" + balance + ", BigRational balance=" + bdValue.toPlainString());
        }
    }
    
    public static void siegfriedRump() {
        System.out.println("Siegfried Rump formula");
        double fpValue;
        {
            double a = 77617.0;
            double b = 33096.0;
            fpValue = 333.75 * Math.pow(b, 6) + a * a * (11.0 * a * a * b * b - Math.pow(b, 6) - 121.0 * Math.pow(b, 4) - 2.0) + 5.5 * Math.pow(b, 8) + a / (2.0 * b);
        }
        
        BigRational brValue;
        {
            BigRational a = BigRational.valueOf(77617);
            BigRational b = BigRational.valueOf(33096);
            BigRational clause1 = BigRational.valueOf(333.75).multiply(b.pow(6));
            BigRational clause2a = BigRational.valueOf(11).multiply(a).multiply(a).multiply(b).multiply(b);
            BigRational clause2b = b.pow(6).add(BigRational.valueOf(121).multiply(b.pow(4))).add(BigRational.valueOf(2));
            BigRational clause2 = a.multiply(a).multiply(clause2a.subtract(clause2b));
            BigRational clause3 = BigRational.valueOf(5.5).multiply(b.pow(8));
            BigRational clause4 = a.divide(b.multiply(BigRational.valueOf(2)));
            brValue = clause1.add(clause2).add(clause3).add(clause4);
        }
        
        System.out.println("  FP value is " + fpValue);
        System.out.println("  BigRational rounded value is " + brValue.toBigDecimal(64, RoundingMode.HALF_UP).toPlainString());
        System.out.println("  BigRational full value is " + brValue.toString());
    }
    
    public static void main(String... args) {
        wrongConvergence();
        
        System.out.println();
        chaoticBankSociety();

        System.out.println();
        siegfriedRump();
    }
}