# Fermion site-type operator \`c\`: \`inner()\` indicates \`c\` on different sites commute

**URL:** https://itensor.discourse.group/t/fermion-site-type-operator-c-inner-indicates-c-on-different-sites-commute/1851
**Category:** ITensor Julia Questions
**Tags:** fermions
**Created:** [July 2, 2024, 8:15pm UTC](https://itensor.discourse.group/t/fermion-site-type-operator-c-inner-indicates-c-on-different-sites-commute/1851 "2024-07-02T20:15:06Z")
**Posts on this page:** 1
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### Author: ![ryanlevy](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/ryanlevy/32/150_2.png) [@ryanlevy](https://itensor.discourse.group/u/ryanlevy)
#### Post date: [July 3, 2024, 11:32am UTC](https://itensor.discourse.group/t/fermion-site-type-operator-c-inner-indicates-c-on-different-sites-commute/1851/2 "2024-07-03T11:32:11Z")

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You should assume that odd numbers of fermion operators will need their strings handled manually (`OpSum` doesn’t support this yet).

Another important thing to note, once you write `c1 = op("c", ss[1])`, then `c1` is an ITensor and has no knowledge of being fermionic as it has no QNs.  
The strings are handled at an operator level, so `OpSum` and some of `correlation_matrix` can insert JW strings.

See the discussion here for some more: [Proper way to construct fermion gates](https://itensor.discourse.group/t/proper-way-to-construct-fermion-gates/172)

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