Most likely there are no generalized rules other than the following rules-of-thumbs: 1. A number with odd number of odd digits is likely to form an additive prime. 2. A number with even number of odd digits will never form an additive prime. 3. A number with even number of odd digits must be accompanied by odd number of odd digits not necessary equal to the former to likely to form additive prime. Here are my Maxima programline to confirm the above results: ggg(n):=f([a:eval_string("n"), x:charlist(string(n)),b:(sum(eval_string(part(x,k)),k,1,length(x)))])*(is(equal(primep(a)*primep(b),true^2))-unknown)/(true-unknown);
sum(ggg(n),n,1,1000);
Output:
f([991,["9","9","1"],19])+f([977,["9","7","7"],23])+f([971,["9","7","1"],17])+f([953,["9","5","3"],17])+f([937,["9","3","7"],19])+f([919,["9","1","9"],19])+ f([911,["9","1","1"],11])+f([887,["8","8","7"],23])+f([883,["8","8","3"],19])+f([881,["8","8","1"],17])+f([863,["8","6","3"],17])+f([829,["8","2","9"],19])+ f([827,["8","2","7"],17])+f([823,["8","2","3"],13])+f([821,["8","2","1"],11])+f([809,["8","0","9"],17])+f([797,["7","9","7"],23])+f([773,["7","7","3"],17])+ f([757,["7","5","7"],19])+f([751,["7","5","1"],13])+f([739,["7","3","9"],19])+f([733,["7","3","3"],13])+f([719,["7","1","9"],17])+f([683,["6","8","3"],17])+ f([661,["6","6","1"],13])+f([647,["6","4","7"],17])+f([643,["6","4","3"],13])+f([641,["6","4","1"],11])+f([607,["6","0","7"],13])+f([601,["6","0","1"],7])+f([599,["5","9","9"],23]) +f([593,["5","9","3"],17])+f([577,["5","7","7"],19])+f([571,["5","7","1"],13])+f([557,["5","5","7"],17])+f([487,["4","8","7"],19])+f([467,["4","6","7"],17])+ f([463,["4","6","3"],13])+f([461,["4","6","1"],11])+f([449,["4","4","9"],17])+f([443,["4","4","3"],11])+f([421,["4","2","1"],7])+f([409,["4","0","9"],13])+f([401,["4","0","1"],5])+ f([397,["3","9","7"],19])+f([379,["3","7","9"],19])+f([373,["3","7","3"],13])+f([359,["3","5","9"],17])+f([353,["3","5","3"],11])+f([337,["3","3","7"],13])+f([331,["3","3","1"],7]) +f([317,["3","1","7"],11])+f([313,["3","1","3"],7])+f([311,["3","1","1"],5])+f([283,["2","8","3"],13])+f([281,["2","8","1"],11])+f([269,["2","6","9"],17])+f([263,["2","6","3"],11]) +f([241,["2","4","1"],7])+f([229,["2","2","9"],13])+f([227,["2","2","7"],11])+f([223,["2","2","3"],7])+f([199,["1","9","9"],19])+f([197,["1","9","7"],17])+f([193,["1","9","3"],13]) +f([191,["1","9","1"],11])+f([179,["1","7","9"],17])+f([173,["1","7","3"],11])+f([157,["1","5","7"],13])+f([151,["1","5","1"],7])+f([139,["1","3","9"],13])+ f([137,["1","3","7"],11])+f([131,["1","3","1"],5])+f([113,["1","1","3"],5])+f([101,["1","0","1"],2])+f([89,["8","9"],17])+f([83,["8","3"],11])+f([67,["6","7"],13])+ f([61,["6","1"],7])+f([47,["4","7"],11])+f([43,["4","3"],7])+f([41,["4","1"],5])+f([29,["2","9"],11])+f([23,["2","3"],5])+f([11,["1","1"],2])+f([7,["7"],7])+f([5,["5"],5]) +f([3,["3"],3])+f([2,["2"],2]) HuneYeong Kong