In that case, it does not work to use the definition "half-life is the time required for exactly half of the entities to decay".
Prva iregularna praštevila so :: 37, 59, 67, 101, 103, 131, 149, 157, 233, 257, 263, 271, 283, 293, 307, ...Nekatera izmed njih so praštevilski dvojčki: {101, 103}, {461, 463}, {617, 619}, {809, 811}, ...Leta 1915 je Jensen dokazal, da je iregularnih praštevil neskončno mnogo.