It CAN not be reduced
"index" is the singular form - "indices" is the plural :)
The possessive form of the singular noun index is index's.
indices or indexes
The plural form for the noun index is indexes.
40.000000 in standard index form is 4.0 × 101
Index form of 9765625 is 9.765625 × 106
5,200 in standard index form = 5.2 × 103
8890 in standard index form = 8.89 × 103
The singular is the noun "index".
Index books is the plural form.
2.9 million in standard index form = 2.9 × 106
250 written in index form = 2.5 × 102