a(1) = 5 because 5 = (3 + 5 + 7)/3.
a(2) = 211 because 211 = (199 + 211 + 223)/3
= (179 + 181 + 191 + 193 + 197 + 199 + 211 + 223 + 227 + 229 + 233 + 239 + 241)/13.
a(3) = 53 because 53 = (47 + 53 + 59)/3 = (41 + 43 + 47 + 53 + 59 + 61 + 67)/7
= (31 + 37 + 41 + 43 + 47 + 53 + 59 + 61 + 67 + 71 + 73)/11.
a(4) = 157 because 157 = (151 + 157 + 163)/3 = (139 + 149 + 151 + 157 + 163 + 167 + 173)/7
= (131 + 137 + 139 + 149 + 151 + 157 + 163 + 167 + 173 + 179 + 181)/11
= (97 + 101 + 103 + 107 + 109 + 113 + 127 + 131 + 137 + 139 + 149 + 151 + 157 + 163 + 167 + 173 + 179 + 181 + 191 + 193 + 197 + 199 + 211 + 223 + 227)/25.
a(5) = 173 because 173 = (167 + 173 + 179)/3 = (157 + 163 + 167 + 173 + 179 + 181 + 191)/7
= (131 + 137 + 139 + 149 + 151 + 157 + 163 + 167 + 173 + 179 + 181 + 191 + 193 + 197 + 199 + 211 + 223)/17
= (109 + 113 + 127 + 131 + 137 + 139 + 149 + 151 + 157 + 163 + 167 + 173 + 179 + 181 + 191 + 193 + 197 + 199 + 211 + 223 + 227 + 229 + 233)/23
= (107 + 109 + 113 + 127 + 131 + 137 + 139 + 149 + 151 + 157 + 163 + 167 + 173 + 179 + 181 + 191 + 193 + 197 + 199 + 211 + 223 + 227 + 229 + 233 + 239)/25.
a(6) = 304517 because 304517 = (304511 + 304517 + 304523)/3
= (304489 + 304501 + 304511 + 304517 + 304523 + 304537 + 304541)/7
= (304481 + 304489 + 304501 + 304511 + 304517 + 304523 + 304537 + 304541 + 304553)/9
= (303691 + ... + 304517 + ... + 305339)/135 = (303649 + ... + 304517 + ... + 305369)/143
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