四棱锥\(P-ABCD\)中,四边形\(ABCD\)为平行四边形,\(AC\)与\(BD\)交于点\(O\),点\(G\)为\(BD\)上一点,\(BG=2GD\),\( \overrightarrow{PA}= \overrightarrow{a}\),\( \overrightarrow{PB}= \overrightarrow{b}\),\( \overrightarrow{PC}= \overrightarrow{c}\),用基底\(\{ \overrightarrow{a}, \overrightarrow{b}, \overrightarrow{c}\}\)表示向量\( \overrightarrow{BG}=\) ______ .