What is black's best sixth move in the Sicilian Grand Prix? 1.e4 c5 2.Nc3 Nc6 3.f4 g6 4.Nf3 Bg7 5.Bb5 Nd4 6.O-O Nxb5 is most common.

[FEN ""]
1.e4 c5 2.Nc3 Nc6 3.f4 g6 4.Nf3 Bg7 5.Bb5 Nd4 6.O-O Nxb5 (6...a6)


Instead of 6...Nxb5 if black plays 6...a6, it seems white has some difficulties. Can anyone give the best move after this position with continuation? If possible give some games after the move 6...a6 which gives a easy play for white.
Things don't look too difficult for White after the natural 7.Bc4 (in your 6...a6 line); not that I think White has some great position, but I at least don't see why you write "it seems White has some difficulties." Since you didn't elaborate on what you think these difficulties are, I'm going to guess that they revolve around White's light-square bishop getting harassed by expanding black pawns. Assuming that is the concern, I don't think there's really any reason to worry.

For instance, if Black immediately lashes out with 7...b5, White can keep the bishop active on the a2-f7 diagonal without fear of getting trapped by ...c4 since he can gain time against Black's rook: 8.Bd5 Rb8 9.d3, when 9...e6 can be safely met with 10.Bb3. After say 10...Bb7, Black has a perfectly good position, but the same goes for White. (Note that White should welcome 10...Nxb3 11.axb3 with good central control and play down the a-file. And in any case White shouldn't mind losing the bishop pair in this position, especially since he played into a line in which Black had the earlier option of 6...Nxb5 anyway.)

Similarly, if Black prepares ...b5 with 7...e6, I think White should simply play 8.d3 with play similar to the above line, e.g. 8...b5 9.Bb3 Bb7, and again White should be content with something like 9...Nxb3 10.axb3 Bb7 as well.

Perhaps I'm wrong about what difficulties you see for White after 6...a6 (in which case please correct me with more detail in your post), but if that is where you think trouble lies, it seems to me that everything is fine for White (though fine for Black too).

[fen ""]

1.e4 c5 2.Nc3 Nc6 3.f4 g6 4.Nf3 Bg7 5.Bb5 Nd4 6.O-O a6 
    ( 6...Nxb5 )
7.Bc4 b5 
    ( 7...e6 8.d3 b5 9.Bb3 Bb7 
        ( 9...Nxb3 10.axb3 Bb7 )
    )
8.Bd5 Rb8 9.d3 e6 10.Bb3 Bb7 
    ( 10...Nxb3 11.axb3 )
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