0.4323320871859028689...

This number is neither prime nor composite.

               0.4323 3208718590 2868909253 7932419999 6370511089 6877651310
3281520671 5855390511 5295886642 4773023467 5307312901 3588747517 1102192547
3474173059 9816815325 2537010284 6860319246 0457044667 2860224884 0679362020
1938436437 9879295524 6786129609 7638935269 4027752231 9731978458 6355957940
3620206633 8633654544 8951089096 5971586278 7332585763 6862001836 7995212808
7865043794 6101266432 6042252640 0822552675 2215113354 1703783531 9471839...

The value of the continued fraction 1/(2+1/(3+1/(5+1/(7+1/(11+....))))). [Hartley]