Abstract
We derive a very short expression for the group inverse of a 1 + ··· + a n when a 1, … , a n are elements in an algebra having group inverse and satisfying a i a j = 0 for i < j. We apply this formula in order to find the group inverse of 2 × 2 block operators under some conditions.
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Acknowledgements
X. Liu and T. Zhu are supported by Guangxi Science Foundation (0640016, 0832084).