A certain man put a pair of rabbits in a place surrounded on all sides by a wall. How many pairs of rabbits can be produced from that pair in a year if it is supposed that every month each pair begets a new pair which from the second month on becomes productive?

The resulting numbers of rabbits, month by month form a series known as the Fibonacci series.

This is the first 100

1

1

2

3

5

8

13

21

34

55

89

144

233

377

610

987

1597

2584

4181

6765

10946

17711

28657

46368

75025

121393

196418

317811

514229

832040

1346269

2178309

3524578

5702887

9227465

14930352

24157817

39088169

63245986

102334155

165580141

267914296

433494437

701408733

1134903170

1836311903

2971215073

4807526976

7778742049

12586269025

20365011074

32951280099

53316291173

86267571272

139583862445

225851433717

365435296162

591286729879

956722026041

1548008755920

2504730781961

4052739537881

6557470319842

10610209857723

17167680177565

27777890035288

44945570212853

72723460248141

117669030460994

190392490709135

308061521170129

498454011879264

806515533049393

1304969544928657

2111485077978050

3416454622906707

5527939700884757

8944394323791464

14472334024676220

23416728348467684

37889062373143904

61305790721611592

99194853094755504

160500643816367104

259695496911122592

420196140727489664

679891637638612224

1100087778366101888

1779979416004714240

2880067194370816000

4660046610375530496

7540113804746346496

12200160415121876992

19740274219868225536

31940434634990100480

51680708854858326016

83621143489848426496

135301852344706760704

218922995834555203584

354224848179261997056

573147844013817200640

927372692193079197696

After that, it gets big. If you left the imaginary rabbits for 1000 months (83 years), you’d wind up with this many:

Which in my day was considered to be a great deal of rabbits.